Revert "feat: Add useCache optional param to getLatestBlock() (#340)"

[jiexi]

This reverts commit 1d1406b.

---

Note

Removes the useCache parameter from getLatestBlock() and updates implementation/tests accordingly; changelog updated.

• API (BREAKING):
  • PollingBlockTracker.getLatestBlock() no longer accepts the optional { useCache } parameter; callers use getLatestBlock() with no args.
• Implementation:
  • Simplifies getLatestBlock() to always return cached value if present or fetch/wait depending on running state; removes delayed pending-promise reset used for rate limiting when not running.
• Tests:
  • Removes all test cases covering { useCache: false } scenarios and adjusts related expectations.
• Docs:
  • Updates CHANGELOG.md under [Unreleased] to note the removal and clarify freshness behavior.

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[Gudahtt]

Instead of removing this flag, we're going to document the bug with this flag and fix it in a later PR.

For now we've removed any usages of this flag instead as a workaround (see MetaMask/eth-json-rpc-middleware#421)