Undeprecate bp_force_buddybar()

The change introduced in r8579 means that bp_force_buddybar() is still used in
a BP core function, and thus should not be deprecated. (If kept in the
deprecated functions file, a fatal error is caused when these files are not
loaded.)

See #5390

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8602 cdf35c40-ae34-48e0-9cc9-0c9da1808c22