[NFC] Remove unused type alias in OpenACC

Leftover from a previous commit, this ends up not being used, so remove
it.

Author: erichkeane

clang/include/clang/Parse/Parser.h

@@ -3611,9 +3611,6 @@ class Parser : public CodeCompletionHandler {
   OpenACCClauseParseResult OpenACCCannotContinue();
   OpenACCClauseParseResult OpenACCSuccess(OpenACCClause *Clause);
 
-  using OpenACCConditionExprParseResult =
-      std::pair<ExprResult, OpenACCParseCanContinue>;
-
   /// Parses the OpenACC directive (the entire pragma) including the clause
   /// list, but does not produce the main AST node.
   OpenACCDirectiveParseInfo ParseOpenACCDirective();