Remove obsolete function call in promise chain

Promise was made obsolete by 206966a5ae8c4ab369213d99df6707e64f377862,
but never completely removed.

Change-Id: I1fde9c281b5a4e446bb31e8a0bac10e4d210672b

polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js

diff --git a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js
index d780a0e..8ed6759 100644
--- a/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js
+++ b/polygerrit-ui/app/elements/change/gr-file-list/gr-file-list.js
@@ -433,12 +433,7 @@
 
     _getFiles() {
       return this.$.restAPI.getChangeFilesAsSpeciallySortedArray(
-          this.changeNum, this.patchRange).then(files => {
-            // Append UI-specific properties.
-            return files.map(file => {
-              return file;
-            });
-          });
+          this.changeNum, this.patchRange);
     },
 
     /**