perf help: No need to use strbuf_remove()

It is the only user of this function, just use the strlen() to skip
the prefix.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-blao710l5cd5hmwrhy51ftgq@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-help.c

@@ -86,8 +86,7 @@ static int check_emacsclient_version(void)
 		return -1;
 	}
 
-	strbuf_remove(&buffer, 0, strlen("emacsclient"));
-	version = atoi(buffer.buf);
+	version = atoi(buffer.buf + strlen("emacsclient"));
 
 	if (version < 22) {
 		fprintf(stderr,