Remove scan_execution_policy_ui feature flag

What does this MR do?

Remove scan_execution_policy_ui feature flag

• the scan_execution_policy_ui feature flag and the security_orchestration_policies_configuration feature flag serve he same purpose, so I am condensing them

Screenshots or Screencasts (strongly suggested)

Title	Screenshot
With :scan_execution_ui disabled and :security_orchestration_policies_configuration disabled
With :scan_execution_ui disabled and :security_orchestration_policies_configuration enabled

How to setup and validate locally (strongly suggested)

1. Navigate to a project
2. Ensure the project has an environment created, or create one by going to Depyloyments => Environments => New Environment
3. Navigate to Security & Compliance => Threat Monitoring => Policies => New Policy
4. Verify a user cannot create a Scan Execution policy
5. Toggle the :security_orchestration_policies_configuration feature flag on with Feature.enable(:security_orchestration_policies_configuration)
6. Refresh the page
7. Verify a user can create a Scan Execution policy

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #273791 (closed)