gnu: Remove linux-libre 5.7.

gnu: Remove linux-libre 5.7.

The 5.7 kernel series is no longer supported upstream.

* gnu/packages/linux.scm (deblob-scripts-5.7, linux-libre-5.7-version,
linux-libre-5.7-pristine-source, linux-libre-5.7-source,
linux-libre-headers-5.7, linux-libre-5.7) Remove variables.
* gnu/packages/aux-files/linux-libre/5.7-arm.conf,
gnu/packages/aux-files/linux-libre/5.7-arm64.conf,
gnu/packages/aux-files/linux-libre/5.7-i686.conf,
gnu/packages/aux-files/linux-libre/5.7-x86_64.conf: Delete files.
* Makefile.am (AUX_FILES): Remove them.