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;