Remove @group from Productivity Improvement template

What does this MR do?

Remove mention of @gl-quality/eng-prod from Productivity Improvement template. This template might be used by public to report bugs or feature requests, which may not need the attention of everyone in the team.

These issues would eventually be included in unlabelled issue triage package and labelled with the appropriate stage, category, group labels.

Example of such issues:

• https://gitlab.com/gitlab-org/gitlab/-/issues/217646
• #217297 (closed)
• #217556 (closed)
• #217182 (closed)

/cc @gl-quality/eng-prod WDYT?