cmCustomCommandGenerator: Simplify cmOutputConverter access

In commit v3.4.0-rc1~480^2~3 (cmCustomCommandGenerator: Port to
cmOutputConverter, 2015-06-04), cmCustomCommandGenerator's access to the
local generator was removed so it needed to construct its own
cmOutputConverter instance.  Access to the local generator was then
restored by commit v3.4.0-rc1~285^2~21 (cmCustomCommandGenerator:
Require cmLocalGenerator in API, 2015-07-25), so now we can use its
cmOutputConverter base class methods directly.

Source/cmCustomCommandGenerator.cxx

@@ -8,7 +8,6 @@
 #include "cmGeneratorTarget.h"
 #include "cmLocalGenerator.h"
 #include "cmMakefile.h"
-#include "cmOutputConverter.h"
 #include "cmStateTypes.h"
 #include "cmSystemTools.h"
 
@@ -167,8 +166,7 @@ void cmCustomCommandGenerator::AppendArguments(unsigned int c,
     if (this->OldStyle) {
       cmd += escapeForShellOldStyle(arg);
     } else {
-      cmOutputConverter converter(this->LG->GetStateSnapshot());
-      cmd += converter.EscapeForShell(arg, this->MakeVars);
+      cmd += this->LG->EscapeForShell(arg, this->MakeVars);
     }
   }
 }