Remove networks images preload

[vbaranov]

Motivation

After returning of old networks menu #2797, networks images preload wasn't removed #2570

Changelog

Remove preload of network images for enhanced networks menu since that menu was disabled for a while and networks menu styles itself

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 eb194103-058e-4689-a97f-6e84f6686571

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 60 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.8%) to 75.641%

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Files with Coverage Reduction	New Missed Lines	%
apps/indexer/lib/indexer/transform/addresses.ex	1	75.0%
apps/block_scout_web/lib/block_scout_web/views/layout_view.ex	2	86.36%
apps/indexer/lib/indexer/block/fetcher.ex	3	83.13%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc/http.ex	7	64.0%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc/web_socket/web_socket_client.ex	47	18.75%

Totals
Change from base Build edb464cc-8f27-47bb-976a-e7c21c530861:	-0.8%
Covered Lines:	5248
Relevant Lines:	6938

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💛 - Coveralls