nixos/matrix-synapse: remove web_client option

[mmilata]

There is web_client_location as a replacement, however I'm reluctant to add it because it makes it easy to run web client on the same domain as homeserver, which is considered security risk. Maybe if matrix-org/synapse#7006 is merged we can add it with a warning?

Motivation for this change

Option was removed in matrix-synapse-0.34.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[mmilata]

cc @Ma27 @Ralith @roblabla

[Ma27]

@GrahamcOfBorg test matrix-synapse

[Ma27]

@mmilata thanks!