shill: cellular: Remove code interacting with old ModemManager.

BUG=chromium:231086
TEST=Tested the following:
1. Build and run unit tests.
2. Run 'mm_debug' on lumpy with Gobi3k modem and link with E362 modem.
3. Run network_3GSmokeTest autotest lumpy with Gobi3k modem and link
   with E362 modem.
4. Verify shill runs properly on lumpy without modem.

Change-Id: Idf537561b4066511979bec2c10cbaf7bcec9b1d8
Reviewed-on: https://gerrit.chromium.org/gerrit/48087
Reviewed-by: Thieu Le <thieule@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/modem_info.h b/modem_info.h
index ae0191f..0205653 100644
--- a/modem_info.h
+++ b/modem_info.h
@@ -84,9 +84,8 @@
 
   typedef ScopedVector<ModemManager> ModemManagers;
 
-  // Register and start new ModemManagers
-  template <class mm> void RegisterModemManager(const std::string &service,
-                                                const std::string &path);
+  // Registers and starts |manager|. Takes ownership of |manager|.
+  void RegisterModemManager(ModemManager *manager);
   ModemManagers modem_managers_;
 
   ControlInterface *control_interface_;