MFC r355936, r356045: remove unused kbd drivers

r355936:
Kill off dummy kbd drivers

As far as I can tell, these are an artifact of times when linker sets
couldn't be empty, otherwise the kernel build would fail due to unresolved
symbols. hselasky fixed this in r268138, and I've audited the kbd portions
to make sure nothing would blow up due to the empty linker set and
successfully compiled+ran a kernel with no keyboard support at all.

Kill them off now since they're no longer required.

r356045:
terasic_mtl: kill off final dummy keyboard driver

The rest were removed in r355936, which speculated that the cause of this
phenomenon was due to an inability to have an empty linker set. The comment
included with this one shows that this was, in fact, not the reason.

Regardless, syscons no longer seems to have an issue with not having any
keyboard drivers and in-fact ignores the keyboard probe anyways.