[heycam/webidl] Remove the RegExp type (#148)

This has no consumers on the platform, at least in Chromium: https://cs.chromium.org/search/?q=RegExp+file:%5C.idl&sq=package:chromium&type=cs

Like the now-dead Date type, if someone actually wanted to use this, they should use `object` plus some prose. But probably nobody should use this.

/cc @nox 

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