Eliminate dead code in code_generator_v8.pm (related to inheritance)

In some cases inheritance involves *parsing* parent interfaces, which is
a major cause of slowness in the IDL compiler.
This eliminates some dead code that does such parsing, simplifying the
rewrite (since we don't need to check this useless code).

Specifically it eliminates:
* BaseInterfaceName :: only one use, which is a nop
* FindSuperMethod :: no uses

No changes to generated code.

Review URL: https://codereview.chromium.org/109883002

git-svn-id: svn://svn.chromium.org/blink/trunk@163399 bbb929c8-8fbe-4397-9dbb-9b2b20218538