drop top-level plots definition as mapping support

[skshetry]

Part of #7093.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e089ee0) 91.41% compared to head (714e77b) 91.42%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9486   +/-   ##
=======================================
  Coverage   91.41%   91.42%           
=======================================
  Files         479      479           
  Lines       37360    37396   +36     
  Branches     5373     5375    +2     
=======================================
+ Hits        34152    34188   +36     
  Misses       2641     2641           
  Partials      567      567           

Impacted Files	Coverage Δ
dvc/schema.py	100.00% <ø> (ø)
dvc/repo/plots/__init__.py	91.78% <100.00%> (-0.06%)	⬇️
tests/func/plots/test_show.py	100.00% <100.00%> (ø)
tests/func/test_dvcfile.py	100.00% <100.00%> (ø)
tests/integration/plots/conftest.py	100.00% <100.00%> (ø)
tests/integration/plots/test_repo_plots_api.py	100.00% <100.00%> (ø)

... and 6 files with indirect coverage changes

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[daavoo]

Let's remember to update https://github.com/iterative/dvcyaml-schema

[skshetry]

Let's remember to update https://github.com/iterative/dvcyaml-schema

yeah, will create an issue for now.