fix(charts): remove ChartThemeDefinition export from index.ts

[dlabrecq]

A simple fix to remove the ChartThemeDefinition export from the ChartTheme index.ts file.

Fixes #2148

[patternfly-build]

PatternFly-React preview: https://2149-pr-patternfly-react-patternfly.surge.sh

[codecov-io]

Codecov Report

Merging #2149 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2149      +/-   ##
==========================================
- Coverage   80.51%   80.49%   -0.03%     
==========================================
  Files         650      649       -1     
  Lines        8234     8225       -9     
  Branches      610      610              
==========================================
- Hits         6630     6621       -9     
  Misses       1308     1308              
  Partials      296      296

Flag	Coverage Δ
#patternfly3	85.22% <ø> (ø)	⬆️
#patternfly4	75.75% <100%> (-0.06%)	⬇️
#patternflymisc	95.68% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
...ents/ChartDonutUtilization/ChartDonutThreshold.tsx	62.82% <ø> (ø)	⬆️
...eact-charts/src/components/ChartAxis/ChartAxis.tsx	90.9% <ø> (ø)	⬆️
...ct-charts/src/components/ChartUtils/chart-theme.ts	62.5% <ø> (ø)	⬆️
...act-charts/src/components/ChartTheme/ChartTheme.ts	100% <100%> (ø)	⬆️

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