Reverts "Remove #if SHELL_ENABLE_METAL checks in iOS code (#51636)"

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Reverts: #51636

Initiated by: jmagman

Reason for reverting: This caused a slew of unexpected Scuba changes b/333039358

Original PR Author: jmagman

Reviewed By: {cbracken}

This change reverts the following previous change:
All physical iOS devices Flutter supports (iOS 12+) can run Metal. The only time Flutter doesn't use Metal afaik is for iOS simulators running < iOS 13, which is covered by a few if (@available(iOS METAL_IOS_VERSION_BASELINE, *)) checks.

engine/shell/platform/darwin/ios/rendering_api_selection.h

Lines 37 to 41 in 8a51e12

	#if TARGET_OS_SIMULATOR
	#define METAL_IOS_VERSION_BASELINE 13.0
	#else
	#define METAL_IOS_VERSION_BASELINE 10.0
	#endif // TARGET_OS_SIMULATOR

Remove hardware checks for physical devices.

Remove shell_enable_metal from the gn files and the #if SHELL_ENABLE_METAL checks in the iOS embedder.

I limited this PR to just iOS, but I imagine it's safe to remove shell_enable_metal everywhere?

engine/shell/platform/darwin/macos/BUILD.gn

Line 18 in 41da00a

enable_metal = shell_enable_metal

engine/tools/gn

Lines 673 to 679 in 41da00a

	if sys.platform == 'darwin' and args.target_os not in ['android', 'fuchsia']:
	# OpenGL is deprecated on macOS > 10.11.
	# This is not necessarily needed but enabling this until we have a way to
	# build a macOS metal only shell and a gl only shell.
	gn_args['allow_deprecated_api_calls'] = True
	gn_args['skia_use_metal'] = True
	gn_args['shell_enable_metal'] = True

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