Remove the cbs creation for n900 plugin, this lead to crashes on n9 ofono init.

For the latest release of ofono for Nemo Mobile, the modem fails to initialize
with the isimodem driver enabled. This is due to a segmentation fault occurring
in the flush of the atoms of the modem in cbs_assembly_free function. This
resulted in a segmentation fault when using n900 driver.

[n900] Remove cbs creation.