osd/: eliminate unnecessary pg_hit_set_history_t::current_info

osd/: eliminate unnecessary pg_hit_set_history_t::current_info

The only field actually relevant from this structure is .begin, which
duplicates the information in hit_set_start_stamp less well.  The problem
is that the starting point of the currently open hit set is ephemeral
state which shouldn't go into the pg_info_t structure.

This also caused 13185  since pg_info_t.hit_set.current_info gets default
constructed with use_gmt = true regardless of the pool setting.  This
becomes a problem in hit_set_persist since the oid is generated using
the pool setting, rather than the use_gmt value in current_info which
is placed into the history list.  That discrepancy then causes a crash
in hit set trim.  There would also be a related bug if the pool setting
is changed between when current_info is constructed and when it is
written out.

Since current_info isn't actually useful, I'm removing it so that we
don't later rely on invalid fields.

Fixes: 13185
Signed-off-by: Samuel Just <sjust@redhat.com>