perf symbol: Remove unused branch_callstack

perf symbol: Remove unused branch_callstack

branch_callstack was added by commit 8b7bad58efb7 ("perf callchain: Support
handling complete branch stacks as histograms") but never used.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20230330131833.12864-2-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>