Remove rcmds.

If they are still needed, you can find them in the net/bsdrcmds port.

This was proposed June, 20th and approved by various committers [1].
They have been marked as deprecated on CURRENT in r320644 [2] on July, 4th.
Both stable/11 and release/11.1 contain the deprecation notice (thanks to
allanjude@).

Note that ruptime(1)/rwho(1)/rwhod(8) were initially thought to be part of
rcmds but this was a mistake and those are therefore NOT removed.

[1] https://lists.freebsd.org/pipermail/freebsd-arch/2017-June/018239.html
[2] https://svnweb.freebsd.org/base?view=revision&revision=320644

Reviewed by: bapt, brooks
Differential Revision: https://reviews.freebsd.org/D12573