nixpkgs manual: remove reference to now defunct nixpkgs monitor

Changed files
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doc
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doc/introduction.md
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The current Nix expressions of the channels are available in the
[`nixpkgs-channels`](https://github.com/NixOS/nixpkgs-channels) repository,
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which has branches corresponding to the available channels. There is also the
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[Nixpkgs Monitor](http://monitor.nixos.org) which keeps track of updates
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and security vulnerabilities.
···
The current Nix expressions of the channels are available in the
[`nixpkgs-channels`](https://github.com/NixOS/nixpkgs-channels) repository,
+
which has branches corresponding to the available channels.