Revert "core: do not leak mount for credentials directory if mount namespace is enabled"

This reverts commits
- 9ae3624889
  "test-execute: add tests for credentials directory with mount namespace"↲
- 94fe4cf255
  "core: do not leak mount for credentials directory if mount namespace is enabled",
- 7241b9cd72
  "core/credential: make setup_credentials() return path to credentials directory",
- fbaf3b23ae
  "core: set $CREDENTIALS_DIRECTORY only when we set up credentials"

Before the commits, credentials directory set up on ExecStart= was kept
on e.g. ExecStop=. But, with the changes, if a service requests a
private mount namespace, the credentials directory is discarded after
ExecStart= is finished.

Let's revert the change, and find better way later.

Addresses the post-merge comment
https://github.com/systemd/systemd/pull/28787#issuecomment-1690614202.

test/test-execute/exec-load-credential-with-mount-namespace.service

@@ -1,9 +0,0 @@
-# SPDX-License-Identifier: LGPL-2.1-or-later
-[Unit]
-Description=Test for LoadCredential=
-
-[Service]
-ExecStart=/bin/sh -x -c 'test "$$(cat %d/test-execute.load-credential)" = "foo"'
-Type=oneshot
-LoadCredential=test-execute.load-credential
-PrivateMounts=yes

test/test-execute/exec-load-credential-with-seccomp.service

@@ -1,9 +0,0 @@
-# SPDX-License-Identifier: LGPL-2.1-or-later
-[Unit]
-Description=Test for LoadCredential=
-
-[Service]
-ExecStart=/bin/sh -x -c 'test "$$(cat %d/test-execute.load-credential)" = "foo"'
-Type=oneshot
-LoadCredential=test-execute.load-credential
-SystemCallFilter=~open_tree move_mount

test/test-execute/exec-set-credential-with-mount-namespace.service

@@ -1,9 +0,0 @@
-# SPDX-License-Identifier: LGPL-2.1-or-later
-[Unit]
-Description=Test for SetCredential=
-
-[Service]
-ExecStart=/bin/sh -x -c 'test "$$(cat %d/test-execute.set-credential)" = "hoge"'
-Type=oneshot
-SetCredential=test-execute.set-credential:hoge
-PrivateMounts=yes

test/test-execute/exec-set-credential-with-seccomp.service

@@ -1,9 +0,0 @@
-# SPDX-License-Identifier: LGPL-2.1-or-later
-[Unit]
-Description=Test for SetCredential=
-
-[Service]
-ExecStart=/bin/sh -x -c 'test "$$(cat %d/test-execute.set-credential)" = "hoge"'
-Type=oneshot
-SetCredential=test-execute.set-credential:hoge
-SystemCallFilter=~open_tree move_mount