PR template: remove mention of `solutions` branch

[JoshDevHub]

Because

With #109, the solutions to the exercises in this repo have been moved from their own branch to the top-level solutions directory in the main branch. Therefore mention of the unused solutions branch should be removed from the PR template.

This PR

• Remove mention of solutions branch in PR template

Issue

Closes #XXXXX

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. Data types exercise: Update spec files
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [ ] If this PR addresses an open issue, it is linked in the Issue section