FCP-101: remove obsolete 10 and 10/100 Ethernet drivers.

Initial commit adding comment to ObsoleteFiles. Each driver will be
removed in a seperate commit to allow later reverts if required.

FCP: https://github.com/freebsd/fcp/blob/master/fcp-0101.md
Reviewed by: jhb, imp
Differential Revision: https://reviews.freebsd.org/D20230