Simplify default argument parsing in `bp_core_fetch_avatar()`.

* Refrain from setting a couple of unnecessary variables.
* Remove comments, which are redundant with the docblock.

Props dcavins.
See #5958.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9124 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Don't `extract()` in `bp_core_fetch_avatar()`, and pass params to 'bp_core_fetch_avatar_no_grav' hook.

Passing the modified parameters to the hook required moving away from the use
of `extract()`. See #5698.

Props dcavins.
Fixes #5958.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9125 cdf35c40-ae34-48e0-9cc9-0c9da1808c22