gnu: Remove linux-libre 6.0.
This kernel series is no longer supported upstream.
* gnu/packages/linux.scm (linux-libre-6.0-version, linux-libre-6.0-gnu-revision,
deblob-scripts-6.0, linux-libre-6.0-pristine-source, linux-libre-6.0-source,
linux-libre-headers-6.0, linux-libre-6.0): Remove variables.
* gnu/packages/aux-files/linux-libre/6.0-arm.conf,
gnu/packages/aux-files/linux-libre/6.0-arm64.conf,
gnu/packages/aux-files/linux-libre/6.0-i686.conf,
gnu/packages/aux-files/linux-libre/6.0-x86_64.conf: Delete files.
* Makefile.am (AUX_FILES): Remove them.