Remove unused UNINITIALIZED_IS_SAFE macro

This macro does not have a definition on MSVC and we have already
avoided all such uses anyway, so this can just be removed completely.

There is one outstanding use of the macro in loopfilter_neon.c, however
this appears to be pointless since the variables are already fully
initialized so we just remove it.

Bug: aomedia:3507
Change-Id: I6a8db44c7c0f09bcaeacd8b5399ab8aa59721da9

aom_dsp/arm/loopfilter_neon.c

diff --git a/aom_dsp/arm/loopfilter_neon.c b/aom_dsp/arm/loopfilter_neon.c
index d12df2a..fd2a50f 100644
--- a/aom_dsp/arm/loopfilter_neon.c
+++ b/aom_dsp/arm/loopfilter_neon.c
@@ -834,7 +834,7 @@
                              const uint8_t *limit, const uint8_t *thresh) {
   uint32x2x2_t p1q0_p0q1, p1q1_p0q0, p1p0_q1q0;
   uint32x2_t pq_rev;
-  uint8x8_t UNINITIALIZED_IS_SAFE(p1p0), UNINITIALIZED_IS_SAFE(q0q1);
+  uint8x8_t p1p0, q0q1;
   uint8x8_t p0q0, p1q1;
 
   // row0: p1 p0 | q0 q1

aom_ports/mem.h

diff --git a/aom_ports/mem.h b/aom_ports/mem.h
index e396842..a70ce82 100644
--- a/aom_ports/mem.h
+++ b/aom_ports/mem.h
@@ -24,16 +24,6 @@
 #define DECLARE_ALIGNED(n, typ, val) typ val
 #endif
 
-/* Indicates that the usage of the specified variable has been audited to assure
- * that it's safe to use uninitialized. Silences 'may be used uninitialized'
- * warnings on gcc.
- */
-#if defined(__GNUC__) && __GNUC__
-#define UNINITIALIZED_IS_SAFE(x) x = x
-#else
-#define UNINITIALIZED_IS_SAFE(x) x
-#endif
-
 #if HAVE_NEON && defined(_MSC_VER)
 #define __builtin_prefetch(x)
 #endif