Fix service file of agl-compositor for guest integration

After the commit  4b4c0f15d1d2a52e636aa22e56cf864d1a8a656b1 of
meta-agl, AGL Demo IVI based container guest was not showing
display.  The container guest need to drop tty dependency.
Before that commit, agl-compositore did not have tty
dependency.  But that commit added new systemd service file,
it has tty dependency.

This patch re-remove tty dependency from agl-compositor.

Bug-AGL: SPEC-4888

Change-Id: I485041b19b89f06df5c26a557546dfd7d8ecaa73
Signed-off-by: Naoto Yamaguchi <naoto.yamaguchi@aisin.co.jp>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/29172
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>