target weight: rm div by topic epochLength

[xmariachi]

Purpose of Changes and their Description

• Fix problem: topics with shorter epochLengths got higher target weights, but there's no justification for this.
• Alignment with whitepaper: target weights are not dependent on epochLength.
• Added / modified tests.
• v3 migration contains a call to this getTargetWeight function. Previous impl of the function has been internalized in the migration.

Are these changes tested and documented?

• If tested, please describe how. If not, why tests are not needed. -- Unit tests fixed
• If documented, please describe where. If not, describe why docs are not needed. -- No need, just aligment with whitepaper.
• Added to Unreleased section of CHANGELOG.md?

[amimart]

LGTM, thx 🙏