Remove nix-install-package support

'nix-install-package' no longer exists in Nix 1.12.

Closes #268.

src/lib/Hydra/View/NixPkg.pm

@@ -1,27 +0,0 @@
-package Hydra::View::NixPkg;
-
-use strict;
-use base qw/Catalyst::View/;
-use Hydra::Helper::CatalystUtils;
-
-sub process {
-    my ($self, $c) = @_;
-
-    $c->response->content_type('application/nix-package');
-
-    my $build = $c->stash->{build};
-
-    my $channelUri = $c->config->{binary_cache_public_uri} // $c->uri_for('/');
-
-    # FIXME: add multiple output support
-    my $s = "NIXPKG1 http://invalid.org/"
-        . " " . $build->nixname . " " . $build->system
-        . " " . $build->drvpath . " " . $build->buildoutputs->find({name => "out"})->path
-        . " " . $channelUri;
-
-    $c->response->body($s);
-
-    return 1;
-}
-
-1;