(PUP-2716) Remove references to dead settings

Commit 30eb9834f introduced two settings, `ssl_client_ca_chain` and
`ssl_server_ca_chain`, which were referenced in code, but were commented
out in defaults.rb. As a result, the settings could never be set, and
always had a nil value.

Commit f99a759db removed the commented out settings from defaults.rb.

This commit removes references to the dead settings.
(PUP-2716) Deprecate ca_chain_file in documentation only

Since the ssl_{server|client}_ca_chain setting was never settable, the
Puppet::SSL::Configuration#ca_chain_file method is not meaningful, and
is deprecated.

This commit deprecates the method in documentation only.
Merge pull request #3241 from joshcooper/ticket/master/PUP-2716-remove-unused-chain-settings

(PUP-2716) Remove references to dead settings