Bug 17557: Koha::Patrons - Move GetAge to ->set_age (and remove SetAge)

Bug 17557: Koha::Patrons - Move GetAge to ->set_age (and remove SetAge)

As said in the previous commit, I considered SetAge as unnecessary and
removed it.

Test plan:
1/ Edit a patron using the different 'Edit' links
2/ Play with the patron category limited to age ranges, and date of
birth
3/ You should get the expected warning if the date of birth is inside
the patron category date range.

To finish:
  prove t/Circulation/AgeRestrictionMarkers.t t/db_dependent/Reserves.t \
        t/db_dependent/Koha/Patrons.t t/db_dependent/Members.t
should return green

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>