Remove unsound null safety options

[elliette]

After #2426, which removed all the unsound null-safety tests, we should also remove everywhere where we allow unsound null safety to be specified.

For example:

webdev/dwds/lib/src/debugging/metadata/module_metadata.dart

Lines 117 to 119 in ae43e8e

	/// True if the module corresponding to this metadata was compiled with sound
	/// null safety enabled.
	final bool soundNullSafety;

[jyameo]

@bkonyi @elliette, just to clarify regarding this bug: do we want to completely remove all references to soundNullSafety, or should we enforce that its value is always true but retain the soundNullSafety variables for backward compatibility and to avoid potential regressions?

[bkonyi]

I think we can remove everything except the actual flag from the argument parser. We should hide the flag and mark it as deprecated just to make sure we're not breaking anyone.