[AST] Remove dead declaration of DeclContext::getCommonParentContext (swiftlang#16855)

Author: rintaro

include/swift/AST/DeclContext.h

@@ -408,10 +408,6 @@ class alignas(1 << DeclContextAlignInBits) DeclContext {
     return false;
   }
 
-  /// Compute a context C such that C is a parent context of A and B.
-  /// If no such context exists, return \c nullptr.
-  static DeclContext *getCommonParentContext(DeclContext *A, DeclContext *B);
-
   /// Returns the module context that contains this context.
   ModuleDecl *getParentModule() const;