core: verify WorkingDirectory= is outside of API VFS only under mount namespacing

The purpose of the check is to prevent leaking API VFS fds
from host into a mount namespace/container. When mountns
is not used at all, the check is pointless and causes
inconvenience. E.g. file managers might need to be spawned
under those directories, and they surely won't run in mountns.

Suggested in https://github.com/systemd/systemd/pull/33454#issuecomment-2186351467
Fixes #33361

src/core/dbus-execute.c

@@ -2799,10 +2799,6 @@ int bus_exec_context_set_transient_property(
                                 if (!path_is_normalized(simplified))
                                         return sd_bus_error_set(error, SD_BUS_ERROR_INVALID_ARGS,
                                                                 "WorkingDirectory= expects a normalized path or '~'");
-
-                                if (path_below_api_vfs(simplified))
-                                        return sd_bus_error_set(error, SD_BUS_ERROR_INVALID_ARGS,
-                                                                "WorkingDirectory= may not be below /proc/, /sys/ or /dev/");
                         }
                 }