Revert "build: Remove explicit libsoup dependency"

the GUri bindings were added past vala 0.48.2:

$ git log -1 --pretty=oneline 3c69e297eda4b287a6a37d30d4bf46b83241a453
3c69e297eda4b287a6a37d30d4bf46b83241a453 glib-2.0: Add GLib.Uri bindings (since 2.66)
$ git describe 3c69e297eda4b287a6a37d30d4bf46b83241a453
0.48.2-145-g3c69e297e

so allowing to build with that makes coding for Geary simpler e.g. for some time.

This reverts commit 1c1912d0.

I perfectly understand if this is closed right away but it would be cool if vala 0.48 support stayed around for at least a bit longer until e.g. Debian catched up with recent vala in at least unstable or experimental to make building/testing on those (and derived distros) a bit simpler (would also unbreak CI).

Currently there's only 0.48:

https://packages.debian.org/search?suite=sid&keywords=valac

Closes: #1248 (closed)