venv: do not export PS1 in activate

PS1 is a global parameter. It does not need (and should not be) exported
to the users environment.

Author: rpigott

Lib/venv/scripts/common/activate

@@ -23,7 +23,6 @@ deactivate () {
 
     if [ -n "${_OLD_VIRTUAL_PS1:-}" ] ; then
         PS1="${_OLD_VIRTUAL_PS1:-}"
-        export PS1
         unset _OLD_VIRTUAL_PS1
     fi
 
@@ -63,7 +62,6 @@ fi
 if [ -z "${VIRTUAL_ENV_DISABLE_PROMPT:-}" ] ; then
     _OLD_VIRTUAL_PS1="${PS1:-}"
     PS1="__VENV_PROMPT__${PS1:-}"
-    export PS1
     VIRTUAL_ENV_PROMPT="__VENV_PROMPT__"
     export VIRTUAL_ENV_PROMPT
 fi