Remove unused operator

src/hb-ot-layout-common-private.hh

diff --git a/src/hb-ot-layout-common-private.hh b/src/hb-ot-layout-common-private.hh
index 4ab4ee5..e054b59 100644
--- a/src/hb-ot-layout-common-private.hh
+++ b/src/hb-ot-layout-common-private.hh
@@ -566,8 +566,6 @@
 
 struct Device
 {
-  inline int operator () (unsigned int ppem_size) const { return get_delta (ppem_size); }
-
   inline int get_delta (unsigned int ppem_size) const
   {
     unsigned int f = deltaFormat;