Removed test function 'luaH_isdummy'

It was not being used anywhere.

ltable.c

@@ -975,6 +975,4 @@ Node *luaH_mainposition (const Table *t, const TValue *key) {
   return mainpositionTV(t, key);
 }
 
-int luaH_isdummy (const Table *t) { return isdummy(t); }
-
 #endif

ltable.h

@@ -59,7 +59,6 @@ LUAI_FUNC unsigned int luaH_realasize (const Table *t);
 
 #if defined(LUA_DEBUG)
 LUAI_FUNC Node *luaH_mainposition (const Table *t, const TValue *key);
-LUAI_FUNC int luaH_isdummy (const Table *t);
 #endif