Remove rust, cargo from dependencies since the latest version of ex_keccak is using precompiled rust

[vbaranov]

Motivation

After updating to ex_abi@0.6.0, which is using ex_keccak@0.7.1 rust is precompiled poanetwork/ex_abi@e484ed7#diff-06572a96a58dc510037d5efa622f9bec8519bc1beab13c9f251e97e657a9d4edR5.
So, installing Cargo, Rust is not a requirement for Blockscout anymore.

Docs update blockscout/docs#131.

Changelog

Remove cargo and rust as dependencies.

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