Remove recentchanges.rc_cur_time from sql statements
Field is unused since many versions, so remove the use in sql
statements, to allow dropping in a later version.
The field must not be reference from INSERT because it has a default
value, which is used, when no value is specified in the statement.
Keeping the field allows easier rollback on version update, when
something gets wrong with the new version.
Existing Comment in tables.sql was added with r101293, was already
disused in r11088