Clean up Release.gpg when switching from signed to unsigned repository

The problem:

Given I have a signed reprepro distribution (sign_with => '0xSOME_KEY')
when I set sign_with to ''
then reprepro wold not delete Release.gpg
and apt on clients would still retrieve Release.gpg
and the packages from this repo will appear to have invalid signatures
and would not be installed.

The solution:

Ensure that Release.gpg is deleted when the distribution is set to unsigned.