The templace in pull_request_template.md is out of date and a documentation rather than a pull request description.

This was not really contriburing a lot towards good pull request descriptions as most of the time it had to be removed first before the actual descrption is written down.
Removed the pull_request_template.md as discussed in PR#4209

Note that there is a follow-up work in progress (and it will eventually result in a pull request) to update the text in other contribution related documents.
But removing this not-really-a-template template is going to produce an immediate improvement into the contribution process.

Author: gkodinov