Remove IfConfigDecl from the AST

The swift-syntax tree retains information about the parsed #if
regions. Drop it from the semantic AST.

include/swift/SIL/SILWitnessVisitor.h

@@ -172,11 +172,6 @@ public:
     // We only care about the contained VarDecls.
   }
 
-  void visitIfConfigDecl(IfConfigDecl *icd) {
-    // We only care about the active members, which were already subsumed by the
-    // enclosing type.
-  }
-
   void visitPoundDiagnosticDecl(PoundDiagnosticDecl *pdd) {
     // We don't care about diagnostics at this stage.
   }