util/buf: remove rz_buf_peek_u16()

Your checklist for this pull request

[ ] I've read the guidelines for contributing to this repository
[ ] I made sure to follow the project's coding style
[ ] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
[ ] I've added tests that prove my fix is effective or that my feature works (if possible)
[ ] I've updated the rizin book with the relevant information (if needed)

Detailed description

Remove unused function

Test plan

CI is green

Closing issues

Closes https://github.com/rizinorg/rizin/issues/4546

rizinorg / rizin