Remove highbd sad from low bitdepth build

340K binary size reduction. Speed 5 6 7 8 9 are bitexact.

Bug: aomedia:2865
Change-Id: I6eea94844dada59df58c08f98f2c630394e7ff7b
diff --git a/aom_dsp/aom_dsp.cmake b/aom_dsp/aom_dsp.cmake
index 7a56223..11cd7e1 100644
--- a/aom_dsp/aom_dsp.cmake
+++ b/aom_dsp/aom_dsp.cmake
@@ -225,6 +225,9 @@
                      "${AOM_ROOT}/aom_dsp/x86/highbd_quantize_intrin_sse2.c"
                      "${AOM_ROOT}/aom_dsp/x86/highbd_subtract_sse2.c"
                      "${AOM_ROOT}/aom_dsp/x86/highbd_variance_sse2.c")
+    list(REMOVE_ITEM AOM_DSP_ENCODER_ASM_SSE2
+                     "${AOM_ROOT}/aom_dsp/x86/highbd_sad4d_sse2.asm"
+                     "${AOM_ROOT}/aom_dsp/x86/highbd_sad_sse2.asm")
   endif()
 
   list(APPEND AOM_DSP_ENCODER_ASM_SSSE3_X86_64