Revert "perf tools: Default to cpu// for events v5"

This reverts commit 50e200f079 ("perf tools: Default to cpu// for
events v5")

The fixup cannot handle the case that
new style format(which without //) mixed with
other different formats.

For example,
group events with new style format: {mem-stores,mem-loads}
some hardware event + new style event: cycles,mem-loads
Cache event + new style event: LLC-loads,mem-loads
Raw event + new style event:
cpu/event=0xc8,umask=0x08/,mem-loads
old style event and new stytle mixture: mem-stores,cpu/mem-loads/

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1412694532-23391-2-git-send-email-kan.liang@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/include/linux/string.h

@@ -1,4 +1,3 @@
 #include <string.h>
 
 void *memdup(const void *src, size_t len);
-int str_append(char **s, int *len, const char *a);