NFC: Remove unused diagnostic

Author: hamishknight

include/swift/AST/DiagnosticsFrontend.def

@@ -196,9 +196,6 @@ ERROR(repl_must_be_initialized,none,
 ERROR(error_doing_code_completion,none,
       "compiler is in code completion mode (benign diagnostic)", ())
 
-WARNING(completion_reusing_astcontext,none,
-        "completion reusing previous ASTContext (benign diagnostic)", ())
-
 ERROR(verify_encountered_fatal,none,
       "fatal error encountered while in -verify mode", ())