perf tools: Ditch rtrim(), use skip_spaces() to get closer to the kernel

No change in behaviour, just using the same kernel idiom for such
operation.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: André Goddard Rosa <andre.goddard@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-a85lkptkt0ru40irpga8yf54@git.kernel.org


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