staging: iio: drop "select IIO_SIMPLE_DUMMY_EVGEN"

Commit e6477000 ("staging:iio:dummy Add event support + fake event
generator") added "select IIO_SIMPLE_DUMMY_EVGEN if [...]". But there
is no Kconfig symbol named IIO_SIMPLE_DUMMY_EVGEN. The select statement
for that symbol is a nop. Drop it.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Jonathan Cameron <jic23@camd.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>