Make ConduitCall always local/in-process

Summary:
Ref T2783. ConduitCall currently has logic to pick a random remote server, but this is ultimately not appropriate: we always want to send requests to a specific server. For example, we want to send repository requests to a server which has that repository locally. The repository tier is not homogenous, so we can't do this below the call level.

Make ConduitCall always-local; logic above it will select ConduitCall for an in-process request or do service selection for an off-host request via ConduitClient.

Test Plan:
  - Browsed some pages using ConduitCall, everything worked.
  - Grepped for removed stuff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D10959