MIPS: cmdline: Remove redundant Kconfig defaults

CMDLINE, CMDLINE_BOOL & CMDLINE_FORCE all explicitly specify default
values that are the same as the default value for their respective types
anyway (ie. n for booleans, and the empty string for strings).

Remove the redundant defaults.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: linux-mips@vger.kernel.org