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

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

This reverts commits
- 9ae3624889b98f75efa6fd0c5f4b4de3eaf328d4
  "test-execute: add tests for credentials directory with mount namespace"↲
- 94fe4cf2557d1f70f20ee02d32f4c2ae6bc1fb3f
  "core: do not leak mount for credentials directory if mount namespace is enabled",
- 7241b9cd72d6e6079d5140cf24c34e78d3cf43cc
  "core/credential: make setup_credentials() return path to credentials directory",
- fbaf3b23ae4aa79110ebd37aada70ce6a044c692
  "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.