xfree86: Remove vestigial lastScrnFlag

Only the mga XAA code ever set this (hence the compat macro), since XAA
is long gone this can go too.

Signed-off-by: Adam Jackson <ajax@redhat.com>
1 job for xf86-cleanup in 1 minute and 59 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed job #23095

00:01:59