Removed erroneous references to PREFIX environment variable

[CKeilbar]

Closes #160748.

This patch removes references to the PREFIX environment variable in the Bash and Zsh integrated terminals.
Without this change, the PS1 prompt for both of these shells contained $PREFIX, which is a common environment variable users tend to set.

To test these changes:

1. Enable the integrated terminal for Bash or Zsh
2. In the terminal, assign a value to a variable called PREFIX
3. Nothing should happen to the prompt

[ghost]

All CLA requirements met.