feat: optimization: use fs.ReadFile (#10987)

While reviewing cpuprofiles of #10985, I noticed a bunch of slice-grows within this function.
Instead of `io.ReadAll` (which does not know the size in advance), use `fs.ReadFile`, which will perform correctly-sized allocations.

### Tests

This function is already covered by tests in `modules/assetfs/layered_test.go`

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

*The decision if the pull request will be shown in the release notes is up to the mergers / release team.*

The content of the `release-notes/<pull request number>.md` file will serve as the basis for the release notes. If the file does not exist, the title of the pull request will be used instead.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10987
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

modules/assetfs/layered.go

@@ -7,7 +7,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"io"
 	"io/fs"
 	"os"
 	"path/filepath"
@@ -97,14 +96,12 @@ func (l *LayeredFS) ReadFile(elems ...string) ([]byte, error) {
 func (l *LayeredFS) ReadLayeredFile(elems ...string) ([]byte, string, error) {
 	name := util.PathJoinRel(elems...)
 	for _, layer := range l.layers {
-		f, err := layer.Open(name)
-		if os.IsNotExist(err) {
+		bs, err := fs.ReadFile(layer, name)
+		if errors.Is(err, fs.ErrNotExist) {
 			continue
 		} else if err != nil {
 			return nil, layer.name, err
 		}
-		bs, err := io.ReadAll(f)
-		_ = f.Close()
 		return bs, layer.name, err
 	}
 	return nil, "", fs.ErrNotExist