desktop-shell: Delete Exposay

Exposay was done as a showcase for what we could do with Weston and an
efficient compositing pipeline. This was mostly with the old
vendor-specific Raspberry Pi backend which could actually process that
many surfaces bypassing the GPU.

Given enough bitrot, Exposay is now pretty exemplary as what _not_ to do
in a Weston shell - particularly the way it manipulates existing
weston_views rather than create its own non-destructive stack.

As it's annoying technical debt, a terrible example to others, and not a
very compelling showcase in 2022, just delete it.

Signed-off-by: Daniel Stone <daniels@collabora.com>