tools/modprobe: Fix odd --remove-holders behavior

tools/modprobe: Fix odd --remove-holders behavior

--remove-holders was not implying --remove, which means that if the user
called `modprobe --remove-holders xe` it would actually try to insert
the xe module. Fix it and spell it out in the man page about one option
implying the other.

Considering i915 is not currently loaded:

Before:
	kmod $ ./build/modprobe -v --dry-run --remove-holders i915
	insmod /lib/modules/6.14.11-1-MANJARO/kernel/drivers/char/agp/intel-gtt.ko.zst
	insmod /lib/modules/6.14.11-1-MANJARO/kernel/drivers/gpu/drm/i915/i915.ko.zst
	kmod $

After:
	kmod $ ./build/modprobe -v --dry-run --remove-holders i915
	kmod $

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/372