Revert "gfs2: Introduce flag for glock holder auto-demotion"

Remove support for glock holder auto-demotion: we've stopped using this
feature, and the complications that the additional code brings can only
introduce additional problems.  (This leaves the __gfs2_glock_dq() split
in place.)

This reverts commit dc732906c2450939c319fec6e258aa89ecb5a63h.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>