[test] Remove unnecessary -c

This crashes the new driver (swiftlang/swift-driver#1570).

Author: hamishknight

test/attr/open.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-build-swift -emit-module -c %S/Inputs/OpenHelpers.swift -o %t/OpenHelpers.swiftmodule
+// RUN: %target-build-swift -emit-module %S/Inputs/OpenHelpers.swift -o %t/OpenHelpers.swiftmodule
 // RUN: %target-typecheck-verify-swift -I %t
 
 import OpenHelpers