proftpd: don't test if the binary exists in the init script

proftpd: 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.

Replace the DAEMON variable, which was used only once, by the full path
of the binary file.

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