target/mips: Drop redundant gen_io_start/stop()

DMTC0 CP0_Cause does a redundant gen_io_start() and gen_io_end() pair,
even though this is done for all DMTC0 operations outside of the switch
statement. Remove these redundant calls.

Fixes: 5dc5d9f055 ("mips: more fixes to the MIPS interrupt glue logic")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Yongbok Kim <yongbok.kim@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>

target/mips/translate.c

@@ -7401,15 +7401,7 @@ static void gen_dmtc0(DisasContext *ctx, TCGv arg, int reg, int sel)
         switch (sel) {
         case 0:
             save_cpu_state(ctx, 1);
-            /* Mark as an IO operation because we may trigger a software
-               interrupt.  */
-            if (ctx->tb->cflags & CF_USE_ICOUNT) {
-                gen_io_start();
-            }
             gen_helper_mtc0_cause(cpu_env, arg);
-            if (ctx->tb->cflags & CF_USE_ICOUNT) {
-                gen_io_end();
-            }
             /* Stop translation as we may have triggered an intetrupt. BS_STOP
              * isn't sufficient, we need to ensure we break out of translated
              * code to check for pending interrupts.  */