Deprecating the lmc(4) driver

[Ed Maste]

The lmc(4) driver supports hardware for a number of legacy interfaces,
including EIA612/613, T1/E1, T3, etc.  The driver's license is ambiguous
and attempts to contact the author failed.

I would like to remove this driver from 12.0, and have a deprecation
notice in review D15182 [1]; I would appreciate hearing from anyone who
is currently using lmc(4) in case it is still providing some value.

[1] https://reviews.freebsd.org/D15182