Remove 'testcookie' hidden field in BP's login forms.

Due to a recent change in WordPress (http://core.trac.wordpress.org/changeset/25045)
that checks the 'testcookie' value as well as a real cookie value, this
broke login attempts from BP's login forms.

This commit removes the 'testcookie' hidden field so BuddyPress can bypass
the cookie check and allows login attempts to work as before.

Fixes #5219.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7485 cdf35c40-ae34-48e0-9cc9-0c9da1808c22