Improve code style in the Scaladoc implementation.

It fixes the following inefficiences or code style violations:

	- Explicit asInstanceOf calls.
	- Boxing symbols instead of using plain NoSymbol.
	- Matching `this` instead of late-binding.
Remove self types check suppression usage from scaladoc.

Also remove private setting not used anymore.
No longer cache all subclass templates.

Instead only cache direct subclasses and compute all known subclasses
as a transitive closure.
Merge pull request #2454 from vigdorchik/scaladoc_fixes

This is a cleanup of scaladoc code.