Remove unnecessary constraints

* Build-Depends: Drop versioned constraint on liblightdm-gobject-dev, lightdm-vala and valac.

Changes-By: deb-scrub-obsolete

Avoid explicitly specifying -Wl,--as-needed linker flag

Changes-By: lintian-brush
Fixes: lintian: debian-rules-uses-as-needed-linker-flag
See-also: https://lintian.debian.org/tags/debian-rules-uses-as-needed-linker-flag.html

Fix some issues reported by lintian

Remove obsolete fields Contact, Name from debian/upstream/metadata (already present in machine-readable debian/copyright).

Fixes: lintian: vcs-field-not-canonical
See-also: https://lintian.debian.org/tags/vcs-field-not-canonical.html