powerpc: Remove last piece of GEMINI

It seems that Adrian is getting old. He removed almost everything of
GEMINI in commit c5365313 ("[POWERPC] Remove the broken Gemini
support") except this piece.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>