perf hists: Remove hist_entry->used, not used anymore

Since hist_entry__delete() nowadays doesn't actually frees anything that
may be in use by the annotation code.

Eventually we will solve this for good by reference counting struct
symbol.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-uldtgljymtrkns0knpiso5op@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>