gnu: Remove linux-libre 4.19.
This kernel series is no longer supported upstream.
* gnu/packages/linux.scm (linux-libre-4.19-version,
linux-libre-4.19-gnu-revision, deblob-scripts-4.19,
linux-libre-4.19-pristine-source, linux-libre-4.19-source,
linux-libre-headers-4.19, linux-libre-4.19, linux-libre-arm-generic-4.19,
linux-libre-arm-omap2plus-4.19): Remove variables.
* gnu/tests/base.scm (%test-linux-libre-4.19): Remove variable.
* gnu/packages/aux-files/linux-libre/4.19-arm.conf,
gnu/packages/aux-files/linux-libre/4.19-arm64.conf,
gnu/packages/aux-files/linux-libre/4.19-i686.conf,
gnu/packages/aux-files/linux-libre/4.19-x86_64.conf: Delete files.
* Makefile.am (AUX_FILES): Remove them.
Change-Id: If79ccbb948975c544b27757104d360539970377c