[JFFS2] Remove dead file histo_mips.h

Its contents were subsumed into compr_rubin.c in a previous
commit, but I forgot to git-rm it.
Signed-off-by: David Woodhouse <dwmw2@infradead.org>

fs/jffs2/histo_mips.h

-#define BIT_DIVIDER_MIPS 1043
-static int bits_mips[8] = { 277,249,290,267,229,341,212,241}; /* mips32 */