Remove the outdated warning in building from source documentation

[larryliu0820]

📚 The doc issue

Here https://docs.pytorch.org/executorch/main/using-executorch-building-from-source.html

We have a warning section

This warning is not necessary since editable mode now works with code like:

python -c "from executorch.exir import CaptureConfig"

We can completely remove this section.

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw