MIPS: Remove legacy MIPS_MACHINE option

The CONFIG_MIPS_MACHINE option is dead code that hasn't been used in
years. The Kconfig option is not selected anywhere, and the
<asm/mips_machine.h> is not included anywhere either.

To make things worse, for years it co-existed with a separate MIPS
machine implementation as <asm/machine.h>. The two defined the
'mips_machine' structure with different fields, and the 'MIPS_MACHINE'
macro with different parameters. The two used the same memory area
(defined by the linker script) to store data, and you could totally use
the two at the same time for all kinds of funny results.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>