treewide: remove unit descriptions

Systemd's `Description` option is a misnomer (as confessed by `man systemd.unit`):
Its value is used by user-facing tools in place of the unit file name, so this option
could have been more aptly named `label` or `name`.
`Description` should only be set if the unit file name is not sufficient for naming a unit.
This is not the case for our services, except for `systemd.services.nb-netns-bridge`
whose description has been kept.

As an example how this affects users, weird journal lines like
```
nb-test systemd[1]: Starting Run clightningd...
```
are now replaced by
```
nb-test systemd[1]: Starting clightning.service...
```

modules/btcpayserver.nix

@@ -123,7 +123,6 @@ in {
         port=${toString cfg.nbxplorer.port}
       '';
     in {
-      description = "Run nbxplorer";
       wantedBy = [ "multi-user.target" ];
       requires = [ "bitcoind.service" ];
       after = [ "bitcoind.service" ];