Fix Removing branch_exceptions in scan execution policy

What does this MR do and why?

This MR partially fixes removing branch exceptions in scan execution policy

Partially resolves #424844 (closed)

Screenshots or screen recordings

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

Result
no exceptions scan execution.mov

How to set up and validate locally

Feature.enable(:security_policies_branch_exceptions)

1. Enable feature flag
2. Go to a project.
3. Go to Secure -> Policies.
4. Create a scan execution policy with rules with branch exceptions
5. Go to policies list and edit newly created policy
6. Change exceptions to no exceptions, make sure branch_exceptions are removed from yaml

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 #424844 (closed)