MIPS: Remove unused TTABLE macro

asm/asm.h contains a TTABLE macro to generate "text tables" which would
appear to be arrays of pointers to strings. It is unused throughout the
kernel tree, so delete the dead code.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20904/
Cc: linux-mips@linux-mips.org