(PUP-3306) Remove deprecated Puppet::SSL::Inventory#serial method

This commit removes the deprecated `serial` method from the
Puppet::SSL::Inventory class. It is replaced by
Puppet::SSL::Inventory#serials.

This commit is part of the Puppet 4 code removal effort.
Merge pull request #3153 from whopper/pup-3306/remove_serial_method

(PUP-3306) Remove deprecated Puppet::SSL::Inventory#serial method