staging: r8188eu: remove BT_COEXIST settings from Makefile

CONFIG_BT_COEXIST is disabled and not used outside of the Makefile.
Remove the settings in the Makefile as well.

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211016172345.23114-3-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>