Actually fix image diff padding/border

go-gitea#12346 was using the wrong
selector which affected only the single image view. Use the correct
selector for image diffs. Removal of border-radius is still worth to
keep to override the global 3px on <img>.

web_src/less/_repository.less

@@ -440,7 +440,6 @@
                 }
 
                 img {
-                    padding: 0;
                     border-radius: 0;
                 }
             }
@@ -1801,7 +1800,8 @@
         &.file-content {
             img {
                 max-width: 100%;
-                padding: 5px 5px 0;
+                padding: 0;
+                border-radius: 0;
             }
             img.emoji {
                 padding: 0;