Feature: Remove unused bytes from core contract's `checkSignatures` method

[mmv08]

This PR:

• Implements https://github.com/safe-global/safe-contracts/issues/687 by removing the bytes parameter from the core contract's and adding the method with the old signature to the CompatibilityFallbackHandler
• Our change for the overloaded checkNSignatures method with the executor address came in handy when adding the backwards compatible method because we can grab the original sender's address and forward it to the method

[coveralls]

Pull Request Test Coverage Report for Build 6707500820

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 95.208%

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Totals
Change from base Build 6705456434:	0.9%
Covered Lines:	371
Relevant Lines:	380

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💛 - Coveralls