Turning on the flag `-Wunused-packages` turns up a bunch of unused packages.
This commit removes them from `gargantext.cabal`.

(Note that `-Wunused-packages` is NOT turned on in this commit.
I tried to turn it on, but occasionally got a bunch of other warnings
which I didn't understand and couldn't get to reproduce reliably,
about packages that were supposedly added via the `-package` or `-package-id` option
(I don't know about that option) but are unused)