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.

X-MFC-With: r355936