Retire lmc(4)

This driver supports legacy, 32-bit PCI devices, and had an ambiguous
license. Supported devices were already reported to be rare in 2003
(when an earlier version of the driver was removed in r123201).

Reviewed by: rgrimes
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15245