namespace: remove redundant .has_prefix=false

The MountEntry's added for EMPTY_DIR work very similarly to the TMPFS ones.
In both cases, .has_prefix is false.  In fact, .has_prefix is false in
*all* the MountEntry's we add except for the access mounts (READONLY etc).

But EMPTY_DIR stuck out by explicitly setting .has_prefix = false.
Let's remove that.

src/core/namespace.c

@@ -265,7 +265,6 @@ static int append_empty_dir_mounts(MountEntry **p, char **strv) {
                         .path_const = *i,
                         .mode = EMPTY_DIR,
                         .ignore = false,
-                        .has_prefix = false,
                         .read_only = true,
                         .options_const = "mode=755",
                         .flags = MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_STRICTATIME,