Compile aom_clear_system_state to nothing on x86 builds

As all MMX codes in libaom are now followed by an emms instruction,
there should be no need for aom_clear_system_state anymore.

To make sure that there is no missing case, this commit experimentally
compiles aom_clear_system_state away. If nothing breaks,
aom_clear_system_state can be removed from the codebase later.

BUG=aomedia:2698

Change-Id: If3cdca5e7bc9fdcaf968c7a0801a340f11262a65
diff --git a/aom_ports/system_state.h b/aom_ports/system_state.h
index 6640839..46797e3 100644
--- a/aom_ports/system_state.h
+++ b/aom_ports/system_state.h
@@ -16,7 +16,7 @@
 
 #if ARCH_X86 || ARCH_X86_64
 void aom_reset_mmx_state(void);
-#define aom_clear_system_state() aom_reset_mmx_state()
+#define aom_clear_system_state()
 #else
 #define aom_clear_system_state()
 #endif  // ARCH_X86 || ARCH_X86_64