Remove dead code: modify_aes_() wrapping deprecated aes_()

[ishaan-arora-1]

modify_aes_() in R/helpers-gg.R (lines 31–33) wraps the deprecated aes_() function. It is never called anywhere in the codebase — the non-deprecated replacement modify_aes() (lines 38–40) exists and is used instead.

The dead function should be removed along with its roxygen comment block.

[jgabry]

Thanks for opening the issue and finding that we never use modify_aes_(). I'm going to reopen this until we merge the PR.