[Concurrency] Remove an unused variable in ActorIsolationChecker::checkLocalCapture.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -3578,10 +3578,6 @@ namespace {
         }
 
         // Otherwise, we have concurrent access. Complain.
-        bool preconcurrencyContext =
-            getActorIsolationOfContext(
-              const_cast<DeclContext *>(getDeclContext())).preconcurrency();
-
         ctx.Diags.diagnose(
             loc, diag::concurrent_access_of_local_capture,
             parent.dyn_cast<LoadExpr *>(),