libweston: rename weston_output_destroy() to weston_output_release()

'release' is a more appropriate name because the function does not free
the underlying memory. The main reason for this is that we need the name
weston_output_destroy() for new API that actually will free also the
underlying memory.

Since the function is only used in backends and external backends are
not a thing, this does not cause libweston major version bump, even
though it does change the ABI. There is no way external users could have
successfully used this function.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>