makedef.pl: Remove redundant set

This ||= is useless, as just above we've already set MULTIPLICITY if
USE_ITHREADS is set.

makedef.pl

@@ -123,9 +123,7 @@ BEGIN
     }
 }
 
-$define{MULTIPLICITY} ||=
-    $define{USE_ITHREADS} ||
-    $define{PERL_IMPLICIT_CONTEXT} ;
+$define{MULTIPLICITY} ||= $define{PERL_IMPLICIT_CONTEXT} ;
 
 if ($define{USE_ITHREADS} && ! $define{WIN32}) {
     $define{USE_REENTRANT_API} = 1;