remove MsgEthereumTx support in TxConfig

[yihuang]

Closes: #711

Description

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #714 (4407a2b) into main (bc1d81c) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #714      +/-   ##
==========================================
- Coverage   57.11%   57.01%   -0.11%     
==========================================
  Files          63       63              
  Lines        5557     5532      -25     
==========================================
- Hits         3174     3154      -20     
+ Misses       2201     2197       -4     
+ Partials      182      181       -1     

Impacted Files	Coverage Δ
encoding/config.go	100.00% <100.00%> (+11.90%)	⬆️

[fedekunze]

Ack pending changelog entry

[yihuang]

Ack pending changelog entry

changelog added.