[MIPS] get_wchan(): remove uses of mfinfo[64]

This array was used to 'cache' some frame info about scheduler
functions to speed up get_wchan(). This array was 1Ko size and
was only used when CONFIG_KALLSYMS was set but declared for all
configs.

Rather than make the array statement conditional, this patches
removes this array and its uses. Indeed the common case doesn't
seem to use this array and get_wchan() is not a critical path
anyways.

It results in a smaller bss and a smaller/cleaner code:

   text    data     bss     dec     hex filename
2543808  254148  139296 2937252  2cd1a4 vmlinux-new-get-wchan
2544080  254148  143392 2941620  2ce2b4 vmlinux~old

Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>