Remove object cache for bp_core_get_user_domain() function.

This caused issues when using a persistent object cache and when the
members directory page slug was changed in the admin area.

Because bp_core_get_user_domain() has its own object cache that is
keyed to each individual user, cache invalidation becomes troublesome.

Since we are already caching the username, this makes caching the
user domain redundant.  Thus, this commit removes the object
cache associated with the bp_core_get_user_domain() function, while
leaving the 'bp_core_get_user_domain_pre_cache' filter in place for
backwards compatibility.

Fixes #5402.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7911 cdf35c40-ae34-48e0-9cc9-0c9da1808c22