Garbage collect mtxpool_lockbuilder, the mutex pool historically used

Garbage collect mtxpool_lockbuilder, the mutex pool historically used
for lockmgr and sx interlocks, but unused since optimised versions of
those sleep locks were introduced.  This will save a (quite) small
amount of memory in all kernel configurations.  The sleep mutex pool is
retained as it is used for 'struct bio' and several other consumers.

Discussed with:	jhb
MFC after:	3 days