package/weston: remove launcher-libseat meson option

This option only exists in main and is not in a release.

Fixes:
output/build/weston-9.0.0/meson.build:1:0: ERROR: Unknown options: "launcher-libseat"

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>