Fix delete "Severity" criteria from Security Policy rule

What does this MR do and why?

This MR resolves 413712

Problem:

• It was impossible to delete "Severity" criteria from Security Policy rule for existing policies

Outcome:

• It is now ipossible to delete "Severity" criteria from Security Policy rule for existing policies

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

DESCRIPTION	VIDEO
BEFORE	https://www.loom.com/share/a60f884828644ce0abf6af62ab5ef61a
NEW Project	new_project
EXISTING Project	existing_project
EXISTING Group	group_existing

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to project or group
2. Secure -> Policies
3. Create new scan result policy if there are none or add severity criteria to existing
4. Edit policy and try to click trash icon and remove severity criteria.

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.

Related to #413712 (closed)