[SCSS Variables Cleanup] Duplicate `$gl-avatar-size`

We currently have a duplicated scss variable in app/assets/stylesheets/framework/variables.scss

Line 265 and 481 are both:

$gl-avatar-size: 40px;

We should likely delete the one on line 265 as it is under the heading /* Misc */ whereas the one on 481 has a more specific heading of /* Avatar */