Remove require in standalone mode

[ForbesLindesay]

This allows people to browserify a module that requires a module that has already been bundled using browserify --standalone. This is becoming increasingly popular now that browserify is seen as an easy way to add es6 support to modules that are used in older versions of node.

[ForbesLindesay]

It's probably worth noting that this method of removing requires should be pretty fast, in contrast to using derequire, which required figuring out the complete scope chain in order to remove requires.

[zertosh]

This is being discussed in https://github.com/substack/node-browserify/pull/1151