Remove redundant test_fileoutput_has_valid_headers.py

[cianc]

Description

By adding a unit test to validate headers for an empty file, file_test.py now covers all cases covered by test_fileoutput_has_valid_headers.py and we can remove that file.

Related Issue

Please link to the issue this PR resolves: #972

Motivation and Context

Removes redundant test code.

How Has This Been Tested?

Ran all existing unit tests and added a new one.

Screenshots (if appropriate):

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[SaboniAmine]

Thanks !