Revert the implementation of grpc_iomgr_run_in_background() for gevent,

which causes duplicate definitions.

src/core/lib/iomgr/iomgr_custom.cc

@@ -72,8 +72,6 @@ void grpc_custom_iomgr_init(grpc_socket_vtable* socket,
   grpc_set_iomgr_platform_vtable(&vtable);
 }
 
-bool grpc_iomgr_run_in_background() { return false; }
-
 #ifdef GRPC_CUSTOM_SOCKET
 grpc_iomgr_platform_vtable* grpc_default_iomgr_platform_vtable() {
   return &vtable;

src/core/lib/iomgr/iomgr_uv.cc

@@ -37,4 +37,6 @@ void grpc_set_default_iomgr_platform() {
   grpc_custom_iomgr_init(&grpc_uv_socket_vtable, &uv_resolver_vtable,
                          &uv_timer_vtable, &uv_pollset_vtable);
 }
+
+bool grpc_iomgr_run_in_background() { return false; }
 #endif