Remove some dead code in importlib, introduced with the PEP 451 patch.

Remove some dead code in importlib, introduced with the PEP 451 patch.

Early in the PEP 451 implementation some of the importlib loaders had
their own _get_spec() methods to simplify accommodating them.  However,
later implementations removed the need.  They simply failed to remove
this code at the same time. :)