[MIPS] Qemu: Remove platform.

The Qemu platform was originally implemented to have an easily supportable
platform until Qemu reaches a state where it emulates a real world system.
Since the latest release Qemu is capable of emulating the MIPSsim and
Malta platforms, so this goal has been reached.  The Qemu plaform is also
rather underfeatured so less useful than a Malta emulation.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>