libweston: fold weston_compositor_tear_down() into weston_compositor_destroy()

The only reason why we have both weston_compositor_tear_down() and
weston_compositor_destroy() is that the only we had to destroy
the log context was keeping weston_compositor alive and calling
weston_log_ctx_compositor_destroy().

After commit "weston-log: replace weston_log_ctx_compositor_destroy()
by weston_log_ctx_destroy()", it's not necessary to keep a zombie
weston_compositor just to be able to call
weston_log_ctx_compositor_destroy().

Fold weston_compositor_tear_down() into weston_compositor_destroy(),
as this split is useless now.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>

doc/sphinx/toc/libweston/compositor.rst

@@ -4,8 +4,8 @@ Compositor
 :type:`weston_compositor` represents the core object of the library, which
 aggregates all the other objects and maintains their state. You can create it
 using :func:`weston_compositor_create`, while for releasing all the resources
-associated with it, you should use :func:`weston_compositor_tear_down`,
-followed by :func:`weston_compositor_destroy` to destroy it.
+associated with it and then destroy it, you should use
+:func:`weston_compositor_destroy`.
 
 Compositor API
 --------------