xenstore: cleanup tdb.c

xenstore: cleanup tdb.c

Remove all unused functions from tdb.c. This will reduce code size of
xenstored and - more important - of xenstore stubdom.

tdb.c hasn't been updated to a newer version since its introduction in
2005. Any backport of bug fixes or update to a new version will need
major work, so there is no real downside to remove not needed code.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>