feat(performance): remove BranchName in /:owner/:repo/commit/:commit

`BranchName` provides the nearest branch of the requested `:commit`.

It's plenty fast on smaller repositories.
On larger repositories like nixpkgs, however, this can easily take 2-3
seconds on a modern machine on a NVMe.

For context, at the time of writing, nixpkgs has over 650k commits and
roughly 250 branches.

`BranchName` is used once in the whole view:
The cherry-pick target branch default selection.

And I believe that's a logic error, which is why this patch is so small.

The nearest branch of a given commit will always be a branch the commit
is already part of. The branch you most likely *don't* want to
cherry-pick to.

Sure, one can technically cherry-pick a commit onto the same branch, but
that simply results in an empty commit.

I don't believe this is intended and even less so worth the compute.

Instead, the cherry-pick branch selection suggestion now always uses
the default branch, which used to be the fallback.

If a user wants to know which branches contain the given commit,
`load-branches-and-tags` exists and should be used instead.

Also, to add insult to injury, `BranchName` was calculated for both
logged-in and not logged-in users, despite its only consumer, the
cherry-pick operation, only being rendered when a given user has
write/commit permissions.

But this isn't particularly surprising, given this happens a lot in
Forgejo's codebase.

routers/web/repo/commit.go

@@ -416,12 +416,6 @@ func Diff(ctx *context.Context) {
 		}
 	}
 
-	ctx.Data["BranchName"], err = commit.GetBranchName()
-	if err != nil {
-		ctx.ServerError("commit.GetBranchName", err)
-		return
-	}
-
 	ctx.HTML(http.StatusOK, tplCommitPage)
 }