tools: remove unused gflags module

Its only dependant was closure-linter but that was removed in commit
f9dd34d3 ("tools: replace closure-linter with eslint").  Remove it.

Fixes: https://github.com/nodejs/node/issues/3217
PR-URL: https://github.com/nodejs/node/pull/3220


Reviewed-By: Trevor Norris <trev.norris@gmail.com>