Update rspamd 2.2 to 2.5

[qknight]

The source for this is rspamd.nix

And this can all be commented out but we need to patch the configuration then.

rspamd bundled ~20.03 2.2
rspamd in nixpkgs 20.09 is 2.5
rspamd upstream is 3.7.4

• [x] check version 2.5 is used
• [x] check if patching is also needed, first glance: no, but needs tests
• [ ] check the groups.conf/rspamd.local.lua/... files are correctly applied
• [ ] check spam filter works and receiving of mails also

[qknight]

With removing modules/services/email/rspamd.nix implementation it uses the rspamd coming with nixpkgs.