perf trace: Ditch __syscall__arg_val() variant, not needed anymore

All callers now can use syscall__arg_val(arg, idx), be it to iterate
thru the syscall arguments while taking into account alignment, or to
get values for other arguments that affect how the current argument
should be formatted (think of fcntl's 'cmd' and 'arg' arguments).

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.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-wm5b156d8kro1r4y3b33eyta@git.kernel.org


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