Potential PHP fatal errors in bp_comment_author_avatar and bp_post_author_avatar

[xknown]

A static analysis tool to detect PHP 7.2 incompatibilities detected that bp_comment_author_avatar and bp_post_author_avatar call the get_avatar function without any arguments (it requires at least 1 argument ​#). This produces a fatal error in PHP versions >= 7.1 (​#)

The code path to reach the get_avatar call is potentially non reachable, but I think it would be good to fix it or maybe remove it if you think this is no longer necessary.

[boonebgorges]

Thanks for the ticket! This code is indeed unreachable, so we can remove it.

[boonebgorges]

In 12177:

Remove unreachable code in deprecated avatar functions.

The checks for the existence of bp_core_fetch_avatar() have not
been needed since [1636], when the function was introduced.

Props xknown.
Fixes #7902.