Remove redundant PGPROC.lockGroupLeaderIdentifier field.

Remove redundant PGPROC.lockGroupLeaderIdentifier field.

We don't really need this field, because it's either zero or redundant with
PGPROC.pid.  The use of zero to mark "not a group leader" is not necessary
since we can just as well test whether lockGroupLeader is NULL.  This does
not save very much, either as to code or data, but the simplification seems
worthwhile anyway.