Remove check for :select_terminus in Indirector

The last class in Puppet that defined `:select_terminus` was removed in https://github.com/puppetlabs/puppet/commit/40ee670c60ed703c38e420462dca78f8ec09d551.

Encapsulated logic for selecting the right terminus based on request (`:select_terminus`) had previously been moved to the `selector` terminus, here: https://github.com/puppetlabs/puppet/commit/10f6cb8969b4d5a933b333ecb01ce3696b1d57d4

Yet we still check for `:select_terminus` it in Puppet::Indirector::Indirection#prepare: https://github.com/puppetlabs/puppet/blob/master/lib/puppet/indirector/indirection.rb#L315-L319

That check can/should be removed, or if we think there are classes out there that still define `select_terminus` add a deprecation warning.