Revert "vtkOStreamWrapper: Support std::string"

This reverts commit 7feab87c4aada7cd286af1cb02462b5d83847276.
The std::basic_string<> forward declaration is not portable
because some C++ standard libraries put basic_string<> in a
sub-namespace inside std and bring it out with "using".
We will need another approach.

Change-Id: I1ac3152fba8a3b1ea7973a2820e7da7f92b43cb9
Merge topic 'core-string' into master

ec258e29 Revert "vtkOStreamWrapper: Support std::string"