Remove always-true skia_enable_flutter_defines (#51423)

Now that Flutter owns the list of defines to add to Skia's build, we don't need to toggle it on or off. This allows Skia to delete the setting (https://skia-review.googlesource.com/c/skia/+/826398)

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

skia/BUILD.gn

@@ -21,13 +21,12 @@ config("skia_public") {
     "SK_CODEC_DECODES_BMP",
     "SK_CODEC_DECODES_WBMP",
   ]
+  defines += flutter_defines
+
   cflags_objcc = []
   if (is_fuchsia || is_linux) {
     defines += [ "SK_R32_SHIFT=16" ]
   }
-  if (skia_enable_flutter_defines) {
-    defines += flutter_defines
-  }
 
   # TODO(zra): Try using this.
   if (skia_enable_optimize_size) {

tools/gn

@@ -489,7 +489,6 @@ def to_gn_args(args):
     gn_args['enable_unittests'] = False
 
   # Skia GN args.
-  gn_args['skia_enable_flutter_defines'] = True  # Enable Flutter API guards in Skia.
   gn_args['skia_use_dng_sdk'] = False  # RAW image handling.
   gn_args['skia_use_sfntly'] = False  # PDF handling dependency.
   gn_args['skia_enable_pdf'] = False  # PDF handling.
@@ -851,7 +850,6 @@ def to_gn_args(args):
 # build are unused. This method is used instead.
 def to_gn_wasm_args(args, gn_args):
   gn_args['is_official_build'] = True
-  gn_args['skia_enable_flutter_defines'] = True
   gn_args['is_component_build'] = False
   gn_args['use_clang_static_analyzer'] = False
   gn_args['is_clang'] = True