How to overwrite default function from child theme

On the twentyfifteen theme, the category pages show the title as “Category: *archive title*”. I am trying to change that functionality so that it instead simply prints the category title without prepending “Category:”.

I have assumed, and correct me if I’m wrong, that the best way to do this is to override the default word press function get_archive_title() located in general-template.php. To do that, I have added the followig to my child theme’s functions.php:

function get_the_archive_title() {
    if ( is_category() ) {
        $title = sprintf( __( '%s' ), single_cat_title( '', false ) );
    } elseif ( is_tag() ) {
        $title = sprintf( __( 'Tag: %s' ), single_tag_title( '', false ) );
    } elseif ( is_author() ) {
        $title = sprintf( __( 'Author: %s' ), '<span class="vcard">' . get_the_author() . '</span>' );
    } elseif ( is_year() ) {
        $title = sprintf( __( 'Year: %s' ), get_the_date( _x( 'Y', 'yearly archives date format' ) ) );
    } elseif ( is_month() ) {
        $title = sprintf( __( 'Month: %s' ), get_the_date( _x( 'F Y', 'monthly archives date format' ) ) );
    } elseif ( is_day() ) {
        $title = sprintf( __( 'Day: %s' ), get_the_date( _x( 'F j, Y', 'daily archives date format' ) ) );
    } elseif ( is_tax( 'post_format' ) ) {
        if ( is_tax( 'post_format', 'post-format-aside' ) ) {
            $title = _x( 'Asides', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-gallery' ) ) {
            $title = _x( 'Galleries', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-image' ) ) {
            $title = _x( 'Images', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-video' ) ) {
            $title = _x( 'Videos', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-quote' ) ) {
            $title = _x( 'Quotes', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-link' ) ) {
            $title = _x( 'Links', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-status' ) ) {
            $title = _x( 'Statuses', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-audio' ) ) {
            $title = _x( 'Audio', 'post format archive title' );
        } elseif ( is_tax( 'post_format', 'post-format-chat' ) ) {
            $title = _x( 'Chats', 'post format archive title' );
        }
    } elseif ( is_post_type_archive() ) {
        $title = sprintf( __( 'Archives: %s' ), post_type_archive_title( '', false ) );
    } elseif ( is_tax() ) {
        $tax = get_taxonomy( get_queried_object()->taxonomy );
        /* translators: 1: Taxonomy singular name, 2: Current taxonomy term */
        $title = sprintf( __( '%1$s: %2$s' ), $tax->labels->singular_name, single_term_title( '', false ) );
    } else {
        $title = __( 'Archives' );
    }

    /**
     * Filter the archive title.
     *
     * @since 4.1.0
     *
     * @param string $title Archive title to be displayed.
     */
    return apply_filters( 'get_the_archive_title', $title );
}

I have made my simple change to the category display, but of course when I attempt to visit the site I get the following cannot redeclare error:

Fatal error: Cannot redeclare get_the_archive_title() (previously declared in /home/content/59/10247459/html/themadmodder/wp-includes/general-template.php:1142) in /home/content/59/10247459/html/themadmodder/wp-content/themes/themadmodder/functions.php on line 65

So my question is: How do I go about overriding the default get_the_archive_title()?

Or, if I’m not using best practices here, how am I supposed to change this functionality?