Add action to BP_XProfile_ProfileData->delete() method

This is helpful for developers who wish to hook stuff in when the
xprofile_delete_field_data() function is called.

Fixes #5111, props geoffroycochard

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7357 cdf35c40-ae34-48e0-9cc9-0c9da1808c22