Remove incorrect super() call

extension.ShowExt.get_parser called super() with the *parent* class,
skipping the parent's own get_parser.  This is quite surprising without
any additional comments to explain the situation.

The only difference between the two get_parsers is the string that
appears in the usage help text (EXT_ALIAS vs EXTENSION).  Rather than
improve the above super() call, this change removes the specialised
ShowExt.get_parser() implementation entirely since it appears the
generic version is sufficient.

Change-Id: I76b901d6a27fbe6f103baf2f03ea17912123a46b
Merge "Remove incorrect super() call"