Remove TRUECOLOR for LVGLImage.py

[PierreRambaud]

Description of the feature or fix

Based on this commit TRUECOLOR usage is now outdated. If I'm not wrong, only RGB8888 should be used for TRUECOLOR and ARGB8888 for TRUE_COLOR_ALPHA

This is what I get when using this tool:

src/assets/background/background_10.c:268:16: error: 'LV_COLOR_FORMAT_TRUECOLOR' undeclared here (not in a function); did you mean 'LV_COLOR_FORMAT_YUV_END'

Notes

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[XuNeo]

Thank you！