Remove phabricator.conduit-uri config setting

Summary:
We have phabricator.conduit-uri in the config setting, but it
is always the phabricator uir appended with '/api'. So we just remove
this setting.

Test Plan:
test arc diff to make sure it still work.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, epriestley
Differential Revision: 71