feat: remove all legacy handlers

[dadamu]

Description

Closes: #566

This PR removes all legacy handlers since it is no longer used.

See: cosmos/cosmos-sdk#7517

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• confirmed the correct type prefix in the PR title
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[codecov]

Codecov Report

Merging #795 (ecfc47e) into master (73a6816) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #795   +/-   ##
=======================================
  Coverage   81.25%   81.25%           
=======================================
  Files          76       76           
  Lines        6503     6503           
=======================================
  Hits         5284     5284           
  Misses        977      977           
  Partials      242      242           

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[RiccardoM]

LGTM