Remove gitlab.po timestamp files from repository

What does this MR do and why?

These artifacts (created by gettext:regenerate) are excluded in .gitignore and only were commited by accident.

Discovered during #385892.

How to set up and validate locally

bin/rake gettext:regenerate
touch /home/peter/devel/gitlab/gdk/gitlab/locale/el_GR/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/vi_VN/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/ta_IN/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/bs_BA/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/nb_NO/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/bn_BD/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/zh_CN/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/fa_IR/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/sv_SE/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/nl_NL/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/sk_SK/gitlab.po.time_stamp
touch /home/peter/devel/gitlab/gdk/gitlab/locale/he_IL/gitlab.po.time_stamp
...

git clean -nd locale

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.