Revert realtime fetcher small skips feature

[vbaranov]

Reverts #2470

Motivation

This feature causes real time fetcher to skip blocks and catchup fetcher never processes them causing increasing the number of chain breaks. Noticed on Mainnet instance. In view of that fact and unclear profit from implementing of #2470 (it still tries to index indexed blocks), it is suggested to revert that PR.

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build e16133fc-8bca-4acc-9885-6ead51faeda1

• 3 of 16 (18.75%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 75.375%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/indexer/lib/indexer/block/catchup/fetcher.ex	3	4	75.0%
apps/indexer/lib/indexer/block/realtime/fetcher.ex	0	12	0.0%

Totals
Change from base Build b2f4df0b-8a6e-490b-815f-3f3071320e57:	-0.04%
Covered Lines:	5430
Relevant Lines:	7204

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💛 - Coveralls