[ASTImporter] Remove duplicated and dead CXXNamedCastExpr handling code.

Summary:
`CXXNamedCastExpr` importing is already handled in the respective `VisitCXXNamedCastExpr` method.
So this code here can never be reached under normal circumstances and we might as well remove it.

This patch shouldn't change any observable behavior of the ASTImporter.

Reviewers: a.sidorin, a_sidorin

Reviewed By: a_sidorin

Subscribers: martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D51110

llvm-svn: 340466