Remove free_domain() and uma_zfree_domain().

Remove free_domain() and uma_zfree_domain().

These functions were introduced before UMA started ensuring that freed
memory gets placed in domain-local caches.  They no longer serve any
purpose since UMA now provides their functionality by default.  Remove
them to simplyify the kernel memory allocator interfaces a bit.

Reviewed by:	cem, kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25937