Revert of SkNx miplevel building

src/opts/SkNx_sse.h

Index: src/opts/SkNx_sse.h
diff --git a/src/opts/SkNx_sse.h b/src/opts/SkNx_sse.h
index a17d988ee7f29e4b2c6460ad8e4f3a96b151f257..c6163b6a8034d5035d910f66fda05094a17973a2 100644
--- a/src/opts/SkNx_sse.h
+++ b/src/opts/SkNx_sse.h
@@ -379,14 +379,6 @@
                                                        _mm_cvttps_epi32(d.fVec))));
 }
 
-template<> inline Sk4h SkNx_cast<uint16_t, uint8_t, 4>(const Sk4b& src) {
-    return _mm_unpacklo_epi8(src.fVec, _mm_setzero_si128());
-}
-
-template<> inline Sk4b SkNx_cast<uint8_t, uint16_t, 4>(const Sk4h& src) {
-    return _mm_packus_epi16(src.fVec, src.fVec);
-}
-
 
 }  // namespace