basic: delete loadavg.h copy

loadavg.h is an internal header of the Linux source repository, and as
such it is licensed as GPLv2-only, without syscall exception.

We use it only for 4 macros, which are simply doing some math calculations
that cannot thus be subject to copyright.
Reimplement the same calculations in another internal header and delete
loadavg.h from our tree.

src/basic/linux/update.sh

@@ -4,11 +4,7 @@ set -eu
 set -o pipefail
 
 for i in *.h */*.h; do
-    if [[ "$i" == "loadavg.h" ]]; then
-        curl --fail "https://raw.githubusercontent.com/torvalds/linux/master/include/linux/sched/$i" -o "$i"
-    else
-        curl --fail "https://raw.githubusercontent.com/torvalds/linux/master/include/uapi/linux/$i" -o "$i"
-    fi
+    curl --fail "https://raw.githubusercontent.com/torvalds/linux/master/include/uapi/linux/$i" -o "$i"
 
     sed -i -e 's/__user //g' -e '/^#include <linux\/compiler.h>/ d' "$i"
 done