linux.mk: target-install: drop extra (invalid) DEPMOD parameter

DEPMOD is already specified in LINUX_MAKE_FLAGS, and is no longer located
in HOST_DIR/usr/sbin after the move to host-kmod, so drop it from here.

Reported-by: Ryan Coe <bluemrp9@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>