fix: validation with a BaseModel field and a custom root type

[PrettyWood]

Change Summary

__root__ key was added by mistake when the value to validate was already of right type

Related issue number

closes #2449

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #2451 (28aa4fb) into master (d619457) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2451   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         5084      5088    +4     
  Branches      1042      1042           
=========================================
+ Hits          5084      5088    +4     

Impacted Files	Coverage Δ
pydantic/main.py	100.00% <100.00%> (ø)
pydantic/json.py	100.00% <0.00%> (ø)
pydantic/types.py	100.00% <0.00%> (ø)
pydantic/fields.py	100.00% <0.00%> (ø)

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[samuelcolvin]

👍 looks good, thanks.