fix(grc20): remove unused interface, fix typo

[leohhhn]

Description

This PR removes a duplicate, unused interface from the GRC20 package, and fixes a typo.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• [x Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.83%. Comparing base (bde8afc) to head (36f7ef4).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2793      +/-   ##
==========================================
+ Coverage   60.80%   60.83%   +0.02%     
==========================================
  Files         563      563              
  Lines       75158    75516     +358     
==========================================
+ Hits        45703    45937     +234     
- Misses      26086    26173      +87     
- Partials     3369     3406      +37     

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (ø)
gno.land	67.21% <ø> (ø)
gnovm	65.59% <ø> (+0.07%)	⬆️
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (+0.35%)	⬆️
tm2	61.95% <ø> (-0.04%)	⬇️

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