Revert "vtkOStreamWrapper: Support std::string"

This reverts commit 54003c65298d33f4f5e70086feac72dd728404f8.
The <string> header on some platforms does not define a working
streaming operator with just <iosfwd> included.  We will need
another approach.

Change-Id: Id060ff87ef7818e66f8aa5bc034b5606993d12e7
Merge topic 'vtkOStreamWrapper-std-string' into master

25b191bf Revert "vtkOStreamWrapper: Support std::string"