remove "by" in the author line of blog post

in loop-index.php you have that filter : avf_show_author_meta search for it – one of the next lines is:

$meta_author  = '<span class="blog-author minor-meta">' . __( 'by', 'avia_framework' ) . ' ';

there is the linking word : by
so my advice is to have a child-theme loop-index.php
edit that line to :

$meta_author  = '<span class="blog-author minor-meta">';

and create a child-theme subfolder called : includes and put a version of that edited loop-index.php inside it.

PS: if you like to have the same on author loop – look at loop-author.php
search there also for the same filter name and change this line in a similar way – proceed with the changed copy in the same way.