Remove checking for locked VGs

Remove checking for locked VGs

A few places were calling a function to check if a
VG lock was held.  The only place it was actually
needed is for pvcreate which wants to do its own
locking (and scanning) around process_each_pv.

The locking/scanning exceptions for pvcreate in
process_each_pv/vg_read can be enabled by just passing
a couple of flags instead of checking if the VG is
already locked.  This also means that these special
cases won't be enabled unknowingly in other places
where they shouldn't be used.