Remove MOJO_ENABLE_HAS_GPU_FOR_APPLE (#55)

This is now always enabled

Author: keith

mojo/mojo_host_platform.bzl

@@ -194,9 +194,8 @@ def _impl(rctx):
             constraints.extend([
                 apple_constraint,
                 "@mojo_gpu_toolchains//:apple_gpu",
+                "@mojo_gpu_toolchains//:has_gpu",
             ])
-            if rctx.getenv("MOJO_ENABLE_HAS_GPU_FOR_APPLE"):
-                constraints.append("@mojo_gpu_toolchains//:has_gpu")
 
     rctx.file("WORKSPACE.bazel", "workspace(name = {})".format(rctx.attr.name))
     rctx.file("BUILD.bazel", """
@@ -215,7 +214,6 @@ mojo_host_platform = repository_rule(
     implementation = _impl,
     configure = True,
     environ = [
-        "MOJO_ENABLE_HAS_GPU_FOR_APPLE",  # NOTE: Will likely be removed in the future
         "MOJO_IGNORE_UNKNOWN_GPUS",
         "MOJO_VERBOSE_GPU_DETECT",
     ],