Remove uses of "timestamp" field for 2.x upgrade

[oalders]

From @mickeyn's comment in #445:

Also to consider: _timestamp seems to be depracated in ES2+
See https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-timestamp-field.html

bash-3.2$ git grep timestamp
lib/MetaCPAN/Document/Favorite.pm:=head2 timestamp
lib/MetaCPAN/Document/Favorite.pm:Sets the C<_timestamp> field to the value of L</date>.
lib/MetaCPAN/Document/Favorite.pm:has timestamp => (
lib/MetaCPAN/Document/Favorite.pm:    timestamp => { path => 'date', store => 1 },
lib/MetaCPAN/Model/User/Account.pm:=head2 timestamp
lib/MetaCPAN/Model/User/Account.pm:Sets the C<_timestamp> field.
lib/MetaCPAN/Model/User/Account.pm:has timestamp => (
lib/MetaCPAN/Model/User/Account.pm:    timestamp => { store => 1 },
lib/MetaCPAN/Model/User/Session.pm:=head2 timestamp
lib/MetaCPAN/Model/User/Session.pm:Sets the C<_timestamp> field.
lib/MetaCPAN/Model/User/Session.pm:has timestamp => (
lib/MetaCPAN/Model/User/Session.pm:    timestamp => { store => 1 },
lib/MetaCPAN/Script/Session.pm:Purges user sessions.  The timestamp field doesn't appear to get populated in

[mickeyn]

this is already done