[core] Simplify createPalette

[oliviertassinari]

Seen in #44059 (comment). The goal is to reduce the memory pressure.

[mui-bot]

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[Janpot]

Great catch. Alternatively

const getDefault = { dark: getDark, light: getLight };

// ...

  ...getDefault[mode](),

could result in less code 🙂

[oliviertassinari]

Yeah ok, hoisting this could be even simpler and have better performance.

Soon or later, we will need to get a lot more serious with performance monitoring. Maybe mui/mui-x#14804 (comment) is the solution.

[oliviertassinari]

Arf, it doesn't work, the test break, it changes the behavior.

Comparing https://tools-public.onrender.com/prod/pages/bundleSizes?circleCIBuildNumber=759455&baseRef=master&baseCommit=a25a365a4c66738f358ecd745bf3727c0ca9d5b6&prNumber=44075 and https://tools-public.onrender.com/prod/pages/bundleSizes?circleCIBuildNumber=759483&baseRef=master&baseCommit=a25a365a4c66738f358ecd745bf3727c0ca9d5b6&prNumber=44075, size seems to be the same.