Remove missed usage of @first variable

With the introduction of `#second` method and friends, we added an
offsets hash which replaced the @first variable, so removing it from the
reset method to avoid creating an unused variable now.

Introduced in bc625080308e4853ae3036f2ad74fe3826e463ef.
Fail early with "Primary key not included in the custom select clause" in find_in_batches

Before this patch find_in_batches raises this error only on second iteration. So you will know about the problem only when you get the batch size threshold.
Merge pull request #9969 from divineforest/fix-find-in-batches

Fail early with "Primary key not included in the custom select clause" i...
Active Record changelog wording and formatting. [ci skip]