Feature: bytecode size reduction by removing public `getChainId` function, making encodeTransactionData private

[mmv08]

This PR is a pre-requisite for future 1.5.0 features like the overloaded checkNSignatures and the module guard.

Code size after all the changes:

Safe 22913 bytes (limit is 24576)
SafeL2 23755 bytes (limit is 24576)

[github-actions[bot]]

Pull Request Test Coverage Report for Build 5473862599

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 93.976%

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Totals
Change from base Build 5455854105:	-0.01%
Covered Lines:	313
Relevant Lines:	328

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💛 - Coveralls