perf tools: Introduce parse_events_terms__purge()

Purges 'struct parse_event_term' entries from a list_head.

Some users need this because they don't allocate space for the list
head, it maybe on the stack or embedded into some other struct.

Next patch will convert users that need just purging and then the
perf_events__free_terms() routine will free the list head as well,
finally being renamed to perf_events_terms__delete().

Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: He Kuang <hekuang@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/n/tip-4w3zl4ifcl0ed0j4bu3tckqp@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>