staging: r8188eu: remove OnAtim prototype

The OnAtim function was removed in commit 6d933902 ("staging: r8188eu:
remove OnAtim"). Remove the prototype as well.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220724153349.138741-2-martin@kaiser.cx


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