[chore] small changes missed in previous dereferencer.GetAccount() PRs (#1467)

* small formatting changes, rewrite fetchRemoteMedia to use separate funcs + use mutex lock correctly

* move url parsing before acquiring mutex locks

* use wrapped mutexes to allow safe unlocking. (previously i did a fucky and passed mutex by value...)

* remove unused code

* use consistent map keying for dereferencing headers/avatars

---------

Signed-off-by: kim <grufwub@gmail.com>

internal/federation/dereferencing/util.go

@@ -1,31 +0,0 @@
-/*
-   GoToSocial
-   Copyright (C) 2021-2023 GoToSocial Authors admin@gotosocial.org
-
-   This program is free software: you can redistribute it and/or modify
-   it under the terms of the GNU Affero General Public License as published by
-   the Free Software Foundation, either version 3 of the License, or
-   (at your option) any later version.
-
-   This program is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-   GNU Affero General Public License for more details.
-
-   You should have received a copy of the GNU Affero General Public License
-   along with this program.  If not, see <http://www.gnu.org/licenses/>.
-*/
-
-package dereferencing
-
-import (
-	"context"
-)
-
-func loadAndCleanup(ctx context.Context, load func(ctx context.Context) error, cleanup func()) error {
-	// whatever happens, clean up when we're done
-	defer cleanup()
-
-	// try and load
-	return load(ctx)
-}