ARM: OMAP: clock: Remove unused mpurate cmdline option

ARM: OMAP: clock: Remove unused mpurate cmdline option

The 'mpurate' option, historically used for specifying an initial
MPU rate at boot, no longer has any effect due to the supporting
code being removed as it was 'wrong and dangerous' [1].

This patch removes the remaining dead code associated with the
__setup macros to avoid confusion and reduce bloat.

[1] https://patchwork.kernel.org/patch/5954631/

Signed-off-by: Andrew Murray <amurray@embedded-bits.co.uk>
Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>