perf ui browser: Free browser->helpline() on ui_browser__hide()

perf ui browser: Free browser->helpline() on ui_browser__hide()

It is allocated at ui_browser__show(), so free it in its counterpart,
ui_browser__hide().

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-g449kvnbcpli4ceyxbe2jp1e@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>