Remove setting of PS1 from awsenv

 - better to add something to .bash_prompt that includes $AWSUSER in PS1

For example, put something like this in .bash_prompt:

```
function parse_aws_user {
    [ -n "$AWSUSER" ] && echo -e "$AWSUSER "
}
```

And use it wherever you set PS1, eg.

```
PS1="# \$(parse_aws_user)\u at \h in \w\n  "
```