Remove SKNX_NO_SIMD from public.bzl iOS build.

This was added with d2ccc772d "Changes to public.bzl to support building
on iOS with objc_library bazel target." [0]. A recent test showed
nothing that breaks by removing this define, so try removing it now.

[0] https://codereview.chromium.org/1639443002

Change-Id: I9938e6a66b29e18332d9e7c4f60e2f2b2a5355ff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/496606
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Ben Wagner <bungeman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>

public.bzl

@@ -767,7 +767,6 @@ def base_defines(os_conditions):
             # IOS
             [
                 "SK_BUILD_FOR_IOS",
-                "SKNX_NO_SIMD",
                 "SK_NO_COMMAND_BUFFER",  # Test tools that use thread_local.
                 "SK_GL",
             ],