Remove url without browser name from tools/signing/download-unsigned-sha256sums-gpg-signatures-from-people-tpo

In tools/signing/download-unsigned-sha256sums-gpg-signatures-from-people-tpo we try to download signatures from URLs that include a directory with the browser name, and also the old type of URLs which does not: https://people.torproject.org/~builder/builds/tbb_version-buildtbb_version_build/file

We should keep it in the maint-12.0 branch, but remove it from the main and maint-12.0-mullvad branches, to avoid downloading signatures for the wrong browser.