Revert "Libwebp build script updates needed for the roll to 1.3.1 (#764)"

This reverts commit 7154eae5b1e161b064996fcc70e8be85298b8574.
diff --git a/build/secondary/third_party/libwebp/BUILD.gn b/build/secondary/third_party/libwebp/BUILD.gn
index 4a25106..912b1d9 100644
--- a/build/secondary/third_party/libwebp/BUILD.gn
+++ b/build/secondary/third_party/libwebp/BUILD.gn
@@ -59,13 +59,6 @@
   configs += [ ":libwebp_defines" ]
 
   sources = [
-    "//third_party/libwebp/sharpyuv/sharpyuv.c",
-    "//third_party/libwebp/sharpyuv/sharpyuv_cpu.c",
-    "//third_party/libwebp/sharpyuv/sharpyuv_csp.c",
-    "//third_party/libwebp/sharpyuv/sharpyuv_dsp.c",
-    "//third_party/libwebp/sharpyuv/sharpyuv_gamma.c",
-    "//third_party/libwebp/sharpyuv/sharpyuv_neon.c",
-    "//third_party/libwebp/sharpyuv/sharpyuv_sse2.c",
     "//third_party/libwebp/src/dec/alpha_dec.c",
     "//third_party/libwebp/src/dec/buffer_dec.c",
     "//third_party/libwebp/src/dec/frame_dec.c",