perf help: No need to use strbuf_remove()

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>