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