Remove obsolete special cases in .pre-process-index-generator.pl.

[jeremyroman]

• HTMLBaseFontElement was removed from the spec in 2013 (whatwg/html@8c58bed).
• HTMLSourceElement was merged in 2016 (whatwg/html@d8f4b79)
• XMLDocument partial interface was removed in 2016 (whatwg/html@523f7a8).

[jeremyroman]

This doesn't currently build, but I believe after #277 it will.