Don't check buddypress()->site_options in bp_get_option()

This reverts the change in r8188. The check in site_options is intended to
provide a speed benefit, but it causes problems when not mirrored properly
in bp_update_option(). We'll take a closer look at this in a future release.

See #4913

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8199 cdf35c40-ae34-48e0-9cc9-0c9da1808c22