Remove duplicate MobjThinker hook call

For some reason, MobjThinker was invoked inside P_MobjSceneryHook even though a call to it was followed right after it in the callee. There's no reason to make a special exception here since it's already doing the same thing, so this patch removes the call inside P_MobjSceneryHook and places the call after the Lua invocation to remove the code duplication.