buildroot

modem-manager: don't test if the binary exists in the init script

The test doesn't make sense. It just exits without any error if the
binary doesn't exist, which is silly.

Drop the MODEMMANAGER_BIN variable, which was never initialized.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>