target-mips: Remove unused gen_load_ACX, gen_store_ACX and cpu_ACX

target-mips: Remove unused gen_load_ACX, gen_store_ACX and cpu_ACX

Remove the functions gen_load_ACX and gen_store_ACX, which appear to have
been unused since they were first introduced many years ago. These functions
were the only places using the cpu_ACX[] array of TCG globals, so remove
that and its accompanying regnames_ACX[] as well.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>