perf script python: Remove dups in documentation examples

Few shell command examples in perf-script-python.txt has few nitpicks
include:

- tools/perf/scripts/python directory listing command is unnecessarily
  repeated.
- few examples contain additional information in command prompt
  unnecessarily and inconsistently.

This commit fixes them to enhance readability of the document.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
Fixes: cff68e58

 ("perf/scripts: Add perf-trace-python Documentation")
Link: http://lkml.kernel.org/r/20170530111827.21732-4-sj38.park@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>