launchers: remove launchers

Moving forward we're going to be supporting libseat and logind as our
only launchers. We're doing this to reduce our maintenance burden,
and security impact.

Libseat supports all our existing use cases, and seatd can replace
weston-launch so we no longer have to carry a setuid-root program.

This patch removes weston-launch, and launcher-direct, leaving only
libseat and logind.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>