Remove extra network call to get change detail on diff view

An observer was causing an extra call to grab the change detail
when it wasn’t needed since it’s already called in _paramsChanged.

Change-Id: I9a9904145e5d88c377cd9d130a7f9a4be67454e3
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
index 3c7fe02..1753448 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
@@ -94,7 +94,6 @@
     ],
 
     observers: [
-      '_getChangeDetail(_changeNum)',
       '_getProjectConfig(_change.project)',
       '_getFiles(_changeNum, _patchRange.*)',
     ],