Removes pull request description

[geofflamrock]

Currently we ask for a description when creating pull requests for the first time for a stack, and put this in above the list of PRs in the stack. This seems like a good idea to get consistency in how a set of PRs are described, but in practice it has tended to be not that useful and is just another thing to answer.

• The context is often slightly different for PRs in a stack, so a single description tends to need to be changed or not used at all.
• A single line of text is generally not enough to provide meaningful context for a set of PRs, the only way around this is to open the config file to put in the right context, but it all has to be in a single string with newline characters.

This PR removes the description, just leaving the list of PRs to be moved around as desired.