gpu: ion: delete ion_system_mapper.c

The mapper abstraction layer was removed before the initial ion
commit, but a stray ion_system_mapper.c file was left in.  Delete
it.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>