[Differential: removing reviewed/reviewers check from the parser]

Summary:
As we've discussed the check is not needed.

Test Plan:
- php -l

Reviewed By: epriestley
Reviewers: jungejason, epriestley
CC: epriestley
Revert Plan:
sure

Other Notes:

Differential Revision: 81

src/applications/differential/parser/commitmessage/DifferentialCommitMessage.php

@@ -403,13 +403,6 @@ class DifferentialCommitMessage {
       }
     }
 
-    if (isset($fields['Reviewers']) && isset($fields['Reviewed By'])) {
-      $this->fail(
-        null,
-        "Commit message contains both 'Reviewers:' and 'Reviewed By:' ".
-        "fields.");
-    }
-
     return $fields;
   }