venv: Suppress warning message when bash hashing is disabled. (GH-17966)

When using python's built-in venv activaton script
warnings are printed when hashing is disabled in
bash or zsh, like;

`bash: hash: hashing disabled`

This output is not really useful to the end-user and has
been disabled in `virtualenv` for long.

This commit is based on:
https://github.com/pypa/virtualenv/commit/28e85bcd80d04b2a7ebce0e1d0b02d432b7e5593