Remove experimental flag of MOTION_VAR

This experiment has been adopted, we can simplify the code
by dropping the associated preprocessor conditionals.

Change-Id: I2dce80e1e1b2116708b6ba9feeacaacc12af8fc4
diff --git a/aom_dsp/variance.c b/aom_dsp/variance.c
index e7ce732..c07f46d 100644
--- a/aom_dsp/variance.c
+++ b/aom_dsp/variance.c
@@ -1043,7 +1043,7 @@
 #endif  // CONFIG_HIGHBITDEPTH
 #endif  // CONFIG_AV1
 
-#if CONFIG_AV1 && CONFIG_MOTION_VAR
+#if CONFIG_AV1
 static INLINE void obmc_variance(const uint8_t *pre, int pre_stride,
                                  const int32_t *wsrc, const int32_t *mask,
                                  int w, int h, unsigned int *sse, int *sum) {
@@ -1363,4 +1363,4 @@
 #endif  // CONFIG_EXT_PARTITION
 #endif  // CONFIG_EXT_PARTITION_TYPES
 #endif  // CONFIG_HIGHBITDEPTH
-#endif  // CONFIG_AV1 && CONFIG_MOTION_VAR
+#endif  // CONFIG_AV1