Removed obsolete checks for compiler 'bool' support

As VTK 6 has dropped support for antique compilers, surely we can assume 'bool' exists in 2013.
Removed VTK_COMPILER_HAS_BOOL checks.  Kept vtkstd_bool with VTK_LEGACY_REMOVE.

Change-Id: I5ae1143eb78470307cf2a5ba5ff8fa77403d4b4d
Merge topic 'purge-obsolete-bool-checks' into master

6bc6866a Removed obsolete checks for compiler 'bool' support