sdl_impl: Erase the SDLJoystick entry after removing a controller

Previously, disconnecting a controller still leaves a null SDLJoystick entry within the vector of SDLJoysticks mapped by GUID.
When a DirectInput device of the same GUID is reconnected, it adds that device to a new port causing non-detectable input.
Furthermore, opening the "Configure" menu would cause yuzu to crash since it first tries to resolve the name of a null SDLJoystick entry that was not removed.
Resolve this by properly erasing the SDLJoystick entry from the vector.