gnu: Remove linux-libre 6.15.
The 6.15 kernel series is no longer supported upstream.
* gnu/packages/linux.scm (linux-libre-6.15-version,
linux-libre-6.15-gnu-revision, deblob-scripts-6.15,
linux-libre-6.15-pristine-source, linux-libre-6.15-source,
linux-libre-headers-6.15, linux-libre-6.15): Remove variables.
* gnu/packages/aux-files/linux-libre/6.15-arm.conf,
gnu/packages/aux-files/linux-libre/6.15-arm64.conf,
gnu/packages/aux-files/linux-libre/6.15-i686.conf,
gnu/packages/aux-files/linux-libre/6.15-riscv.conf,
gnu/packages/aux-files/linux-libre/6.15-x86_64.conf: Delete files.
* Makefile.am (AUX_FILES): Remove them.
Change-Id: I89fb56d4153589da7205d55458e1c4d8ab53c206