Remove pull_request_template.md

The template in pull_request_template.md is out of date and a documentation
rather than a pull request description. This was not really contributing
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.

This is a continuation of https://github.com/MariaDB/server/pull/4209. The consensus in that PR's discussion is that this file should be removed. Hence I've created https://github.com/MariaDB/server/pull/4487 to actually remove the file.