core/exec-credential: stop removing empty credentials dir

core/exec-credential: stop removing empty credentials dir

Starting from cfbf7538d87023840c5574fa5b0452e5b0f42149 we'd always
install the credentials dir regardless of whether it's empty,
with the correct permissions. Hence the problem stated in the comment
should no longer be a concern. Moreover, this ensures later in
setup_namespace() the mountpoint would be in-place. This is important
for credential reloading as it saves the trouble of remounting
the upper tmpfs as rw again and create the mountpoint.