Reinstates $admin_mail variable when compiling registration emails

The admin_email value was originally used to create a custom email header,
which is no longer used. (See [7463] #5186.) However, the $admin_email variable
is still passed to the apply_filters() calls at the end of each filter
function, so the variable must be defined.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7484 cdf35c40-ae34-48e0-9cc9-0c9da1808c22