Remove build badge from Readme.

[dennisoelkers]

Description

Motivation and Context

This PR is removing the build badge from the Readme, as it points towards an action which is not used to build the default branch anymore, but results in pointing to the last PR build, which is kinda random and shows build failures often.

/nocl

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

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

Checklist:

• 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 document.
• I have added tests to cover my changes.