MIPS: MT: Remove SMTC support

Nobody is maintaining SMTC anymore and there also seems to be no userbase.
Which is a pity - the SMTC technology primarily developed by Kevin D.
Kissell <kevink@paralogos.com> is an ingenious demonstration for the MT
ASE's power and elegance.

Based on Markos Chandras <Markos.Chandras@imgtec.com> patch
https://patchwork.linux-mips.org/patch/6719/ which while very similar did
no longer apply cleanly when I tried to merge it plus some additional
post-SMTC cleanup - SMTC was a feature as tricky to remove as it was to
merge once upon a time.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/r4kcache.h

@@ -43,11 +43,10 @@
 	: "i" (op), "R" (*(unsigned char *)(addr)))
 
 #ifdef CONFIG_MIPS_MT
-/*
- * Temporary hacks for SMTC debug. Optionally force single-threaded
- * execution during I-cache flushes.
- */
 
+/*
+ * Optionally force single-threaded execution during I-cache flushes.
+ */
 #define PROTECT_CACHE_FLUSHES 1
 
 #ifdef PROTECT_CACHE_FLUSHES