Add --no-rehash to the pyenv init command, which was removed in [#4492].

The rehash was likely disabled because it can affect shell startup times.
It should only be necessary when installing or removing Python versions.

See [pyenv/pyenv#784] and [sorin-ionescu/prezto#1603] for more detail.

[#4492]: https://github.com/ohmyzsh/ohmyzsh/pull/4492
[pyenv/pyenv#784]: https://github.com/pyenv/pyenv/issues/784
[sorin-ionescu/prezto#1603]: https://github.com/sorin-ionescu/prezto/pull/1603