qsyncthingtray: build with native browser to allow for Qt > 5.6

[peterhoeg]

Motivation for this change

This means we can drop the Qt 5.6 dependency. I've been running with it here for some time without problems.

Cc: @zraexy

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[adisbladis]

@peterhoeg Is there a reason why you would want to build against Qt 5.6? I would like it if we could drop the Qt 5.6 dependency altogether.

[peterhoeg]

This PR removes functionality so if someone desperately needs the built-in browser, they can build it against 5.6.