perf util: Remove kernel version deadcode

perf util: Remove kernel version deadcode

fetch_kernel_version() has been unused since Ian's 2023 commit
3d6dfae889174340 ("perf parse-events: Remove BPF event support")

Remove it, and it's helpers.

I noticed there are a bunch of kernel-version macros that are also
unused nearby.

Also remove them.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241116155850.113129-1-linux@treblig.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>