Remove ResourceLoaderGetStartupModules hook

Deprecated only since 1.23, but universally reviled, and used only by
MwEmbedSupport, which no longer uses it as of I43af3c87a).

Change-Id: I36c89b273fdb0ec3a76034c7a6d2f48a15d5457f
Merge "Remove ResourceLoaderGetStartupModules hook"