Fix header verification after equivalent proofs

[sstanculeanu]

Reasoning behind the pull request

• leader sig should not be removed while checking the header signature

Proposed changes

• avoid removing the leader signature

Testing procedure

• with feat branch

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?