Hotfix : Remove persistence of PC doc comments.

Comments obtained by parsing are cleared at each typer run.

- fixes memory leak
- makes Scaladoc slower

It's possible (and easy : add `icUnit.scalaProject.presentationCompiler(_.clearDocComments())` right before[here](https://github.com/scala-ide/scala-ide/blob/master/org.scala-ide.sdt.core/src/org/scalaide/ui/internal/reconciliation/ScalaReconcilingStrategy.scala#L48)) to reset those comments only at reconciliation,
but the memory-leak test doesn't trigger reconciliation (it types all the time).