Drop tree_orig_create_time

I originally wanted to fix this column (length 12 but attempts
to store length 14 wfTimestampNow)
As Erik pointed out though, this column is useless:
tree_rev_descendant_id holds a uuid, from which the timestamp
can be fetched.
Also, this column isn't even used anywhere.

Change-Id: I504b6792cdad85a5f658f11e0f80553396b1d001
Merge "Drop tree_orig_create_time"