Remove all the legacy handlers

[dadamu]

Description

This PR is implementation of #566.
Closes #566.

Checklist

• Targeted PR against correct branch.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Wrote unit tests.
• Wrote integration tests (simulation & CLI).
• Updated the documentation.
• Added an entry to the CHANGELOG.md file.
• Re-reviewed Files changed in the Github PR explorer.

[codecov]

Codecov Report

Merging #569 (98d0883) into master (71960c8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #569   +/-   ##
=======================================
  Coverage   15.70%   15.70%           
=======================================
  Files         168      168           
  Lines       45954    45954           
=======================================
  Hits         7218     7218           
  Misses      38119    38119           
  Partials      617      617           

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[dadamu]

Sim tests will be solved once cosmos/cosmos-sdk#9139 is out.
Currently, message still pass via legacy route since the current message doesn't implement ServiceMsg struct to get the MethodName for msgServiceRouter.Handler(), In addition, the struct will be deprecated in the next version.

[RiccardoM]

Closing as it's referring to old code that is no longer there