octave: build without qscintilla by default

[bcdarwin]

• qscintilla is used for IDE features in the GUI, which is not built since qt support isn't enabled
• this removes qt from the Octave closure

Motivation for this change

The default Octave build was pulling in qt4 via qscintilla for no reason.

I didn't use any conditional/assert logic in the Octave expression to enforce reasonable dependencies between Qt and Qscintilla, rather simply improving the options used to build Nixpkgs, but I am open to doing so.

Build is still broken on OS X for unrelated reasons; reverse dependency octaveHg won't rebuild due to texlive issues.

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.

[lsix]

Nice catch.

Thanks !