remove references to nonexistent PragmaTable interface

[mostynb]

Summary

There does not appear to be a PragmaTable interface in the repository, either on tip-of-tree or historically.

Let's remove these references.

clang/include/clang/Lex/Pragma.h

 //
 //===----------------------------------------------------------------------===//
 //
-// This file defines the PragmaHandler and PragmaTable interfaces.
+// This file defines the PragmaHandler interface.
 //
 //===----------------------------------------------------------------------===//
 

clang/lib/Lex/Pragma.cpp

 //
 //===----------------------------------------------------------------------===//
 //
-// This file implements the PragmaHandler/PragmaTable interfaces and implements
+// This file implements the PragmaHandler interface and implements
 // pragma related methods of the Preprocessor class.
 //
 //===----------------------------------------------------------------------===//