Remove kernel .map files

Originally created by @anonym on #10951 (Redmine)

We currently ship files like /boot/System.map-${KERNEL_VERSION}-${ARCH} which contains the memory addresses of kernel functions, making the life of malware a bit easier.

We should remove these, since their only purpose is kernel debugging (right?), and they increase the Tails image size (uncompressed the two we have are ~4.5 MiB).

Feature Branch: feature/10951-remove-kernel-map-files

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