Silence gettext_extractor only if '--silent' is passed

What does this MR do and why?

Before this commit this script was always silent.

Reason:

p 'x' in ['x']

This ☝ always returns x because it's Ruby's pattern matching syntax and not Array#include? or x in list in Python.

Follow-up of !124843 (comment 1827257758).

MR acceptance checklist

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How to set up and validate locally

$ tooling/bin/gettext_extractor
Please provide a target file name as the first argument, e.g.
  tooling/bin/gettext_extractor locale/gitlab.pot

$ tooling/bin/gettext_extractor --silent
Please provide a target file name as the first argument, e.g.
  tooling/bin/gettext_extractor locale/gitlab.pot

$ tooling/bin/gettext_extractor locale/gitlab.pot
Extracting translatable strings from source files...
All done. Please commit the changes to `locale/gitlab.pot`.

$ tooling/bin/gettext_extractor locale/gitlab.pot --silent

$ tooling/bin/gettext_extractor --silent locale/gitlab.pot