kbuild: remove a tag file before it is regenerated

If a tag file is not removed before it is regenerated, the newly
generated data is appended to the old, which preserves stale data and
makes the tag file grow over time.

Signed-off-by: Matt Kraai <kraai@ftbfs.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>