Makefile: Remove echoing of kvmtools version file

On every build we report the kvmtool "version" number, which isn't
meaningful at all anymore.

Remove the line from the KVMTOOLS-VERSION-GEN script to drop a
pointless message.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>