[Fix] require-default-props: fix config schema

[controversial]

Fix #3604

Removes extraneous “allow” keyword from JSON schema definition. I couldn’t find any references to this keyword in JSON schema, nor documentation on its purpose, but it seems to break the TypeScript definitions generated in projects like eslint-define-config.

If the “allow” keyword has a purpose here that I’m not aware of, please feel free to close this PR.

[codecov]

Codecov Report

Merging #3605 (1a3a17a) into master (31282dd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3605   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files         132      132           
  Lines        9295     9295           
  Branches     3397     3397           
=======================================
  Hits         9074     9074           
  Misses        221      221           

Impacted Files	Coverage Δ
lib/rules/require-default-props.js	100.00% <ø> (ø)

[ljharb]

This does match the intention in #3249 from what I can tell (also looking at the tests). Thanks!