Core: change `bp_disable_profile_sync()`'s default value to false.

The default value is only used if the site is somehow missing the `bp-disable-profile-sync` option
in the database. The default value for the `bp-disable-profile-sync` option is false, so having the
function default to disabling profile sync if the option is missing is inconsistent.

Fixes #5725

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8553 cdf35c40-ae34-48e0-9cc9-0c9da1808c22