doc: Remove references to third-party maintained bitcoin-core packages

The list is incomplete and needs constant maintenance. For example, it is missing:

* https://tracker.debian.org/pkg/bitcoin (deprecated)
* https://packages.fedoraproject.org/pkgs/bitcoin-core/
* arch: bitcoin/bitcoin#17343 (comment)
* ...

So just remove all third-party ones.

Author: MarcoFalke

README.md

@@ -10,8 +10,6 @@ Bitcoin Core is also distributed on several Linux package managers:
 
 * The `bitcoin-core` snap package for https://snapcraft.io/bitcoin-core is maintained in [/snap](/snap)
 * The `org.bitcoincore.bitcoin-qt` flatpak package is maintained under their organization: https://github.com/flathub/org.bitcoincore.bitcoin-qt
-* The `bitcoin-core` guix package is maintained under https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/finance.scm
-* The `bitcoin` gentoo package is maintained under https://gitlab.com/bitcoin/gentoo
 
 An RPM spec file was removed in commit fa7c698cb24bab350b40ee2d825c443dabdd9633, because is was unmaintained and
 outdated.