libweston/backend-headless: Remove cleanup_after_cairo()

In some circumstances (like system load) we seem to be racing with the
threads fontconfig creates and the resource release happening inside
cleanup_after_cairo(), and we would still find a cached entry holding a
font map reference while we are on the compositor exit path --
which happens in cleanup_after_cairo() when calling
cairo_debug_reset_static_data().

This was introduced with commit 823580e0, 'backend-headless: fully
release pango and fontconfig', as a way to have a clean memory leak
report, but due to the fact we can't influence how libraries manage
their threads, the solution (for now at least) would be to just remove
it entirely.

Running it at the end in the test itself, or before calling exit(2),
while it does narrow the window, it still exhibits the cairo crash assert
related to having cached entries in that font map hash table.

Reference: #756



Signed-off-by: Marius Vlad <marius.vlad@collabora.com>