Refactor pull-request compare&create page (#33071)

The old code is unnecessarily complex.

web_src/css/repo.css

@@ -825,10 +825,6 @@ td .commit-summary {
   height: 10px;
 }
 
-.repository.compare.pull .show-form-container {
-  text-align: left;
-}
-
 .repository .choose.branch {
   display: flex;
   align-items: center;
@@ -866,11 +862,6 @@ td .commit-summary {
   margin-top: -8px;
 }
 
-.repository.compare.pull .pullrequest-form {
-  margin-top: 16px;
-  margin-bottom: 16px;
-}
-
 .repository.compare.pull .markup {
   font-size: 14px;
 }