[doc][webos] Remove outdated flatc section

[neo1973]

Description

With #23253 merged this workaround shouldn't be necessary anymore. At least I think so 😉

Motivation and context

Simplify build instructions.

How has this been tested?

Hasn't been tested at all. @sundermann, could you please verify that this workaround isn't necessary anymore?

What is the effect on users?

None.

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[sundermann]

I never ran into this issue because I'm running Ubuntu 22.04 which has a fairly old flatc version anyway. But it seems fine to remove these instructions after #23253