perf path: Remove unused is_executable_file()

perf path: Remove unused is_executable_file()

is_executable_file() has been unused since 2022's commit
7391db6459388d47 ("perf test: Refactor shell tests allowing subdirs")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Carsten Haitzler <carsten.haitzler@arm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241222215831.283248-1-linux@treblig.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>