This CL removes code that is now unused since the port of regexp.js has been
completed. Removed functions / classes are:

* regexp.js (GetSubstitution moved to string.js)
* RegExpConstructResult stub
* RegExpFlags intrinsic
* RegExpSource intrinsic
* RegExpInitializeAndCompile runtime function

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2448463002
Cr-Commit-Position: refs/heads/master@{#40547}