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:
28e85bcd80

Lib/venv/scripts/common/activate

@@ -18,7 +18,7 @@ deactivate () {
     # be called to get it to forget past commands.  Without forgetting
     # past commands the $PATH changes we made may not be respected
     if [ -n "${BASH:-}" -o -n "${ZSH_VERSION:-}" ] ; then
-        hash -r
+        hash -r 2> /dev/null
     fi
 
     if [ -n "${_OLD_VIRTUAL_PS1:-}" ] ; then
@@ -62,5 +62,5 @@ fi
 # be called to get it to forget past commands.  Without forgetting
 # past commands the $PATH changes we made may not be respected
 if [ -n "${BASH:-}" -o -n "${ZSH_VERSION:-}" ] ; then
-    hash -r
+    hash -r 2> /dev/null
 fi