You can delete time and key signature changes in a project, but you can’t delete the initial time and key signature.
Delete a signature
Do one of the following:
Select the signature in the Signature track, Signature List, or Score Editor, then choose Edit > Delete (or press Delete).
Tip: To erase all time signature events, select a signature, choose Edit > Select Similar Objects, then press Delete.
Click the signature with the Eraser tool in the Signature track, Signature List, or Score Editor.