Delete core_userdata object cache when updating settings

When updating settings, the settings are sometimes saved on the same pageload
as the settings page is displayed. To ensure that the current pageload's fields
reflect just-changed settings, the persistent object cache key for the saved
settings must be manually busted just after a successful save.

Fixes #5098

Props wpdennis

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7586 cdf35c40-ae34-48e0-9cc9-0c9da1808c22