Don't call start on tracking drivers;

Start is mainly used for setting up graphics-related stuff, since it
was created to perform setup after the window/graphics module is
initialized.  Since the display driver is the only one doing rendering,
it makes sense to only call start on the display driver.

...also fixes a bug where start is getting called twice.

src/api/l_headset.c

@@ -115,9 +115,6 @@ static Device luax_optdevice(lua_State* L, int index) {
 
 static int l_lovrHeadsetStart(lua_State* L) {
   lovrHeadsetDisplayDriver->start();
-  FOREACH_TRACKING_DRIVER(driver) {
-    driver->start();
-  }
   return 0;
 }