Maybe someone with more expertise than me can assist you in removing just the “BY ADMIN IN” part, but adding the following code to your Jetpack CSS editor (or whatever you’re editing your CSS in) should allow you to keep just the background:
.fleft {visibility: hidden;}
In general, every theme has its own way of handling things, so probably not. But for the ‘Twenty’ themes, that may work.
Not sure if it’s exactly what you’re looking for, but adding this to Jetpack’s Custom CSS (or your child theme, or whatever you’re editing in) should move the link down there with the rest of the meta info.
#content .post { position: relative; }
#content .comments-link { position: absolute; bottom: 1em; text-align: right; }
#content .post { padding-bottom: 3em; }
