Remove reference to AUTHORS.txt file

[dhodder]

The COPYRIGHT file should be updated to note that Rust no longer ships with AUTHORS.txt.

$ git log -1 -- AUTHORS.txt
commit 402749c539edcbc2d850ac3a782cace8661c68e6
Author: Brian Anderson <banderson@mozilla.com>
Date: Wed Dec 2 22:16:08 2015 +0000
Remove AUTHORS.txt and add-authors.sh
Keeping this file up to date requires hours of work every release,
even with the script. It is a fool's errand and we shall not do it
any longer.

[rust-highfive]

r? @brson

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[brson]

@bors r+ rollup

[bors]

📌 Commit c331ed0 has been approved by brson

[bors]

⌛ Testing commit c331ed0 with merge 332e345...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt