Update the PR table in the contributing guide

[javiereguiluz]

As pointed by @vladapopster, the contributing guide (http://symfony.com/doc/current/contributing/code/patches.html#make-a-pull-request) contains a PR table which is not updated to the latest one used in the Symfony repository (it lacks the Branch row for example).

[wouterj]

Now we have PR templates on github, do we actually need to duplicate the template in the contributing docs? It seems like it can only get outdated and doesn't add anything more than saying "Fill in the table in the PR template."

[xabbuh]

I agree with Wouter. Let's just remove the table from the docs.

[javiereguiluz]

This can be closed as "fixed" because #6365 was merged. Thanks!

[mvar]

Are there any notes why Issue and Pull Request templates were removed? @wouterj wrote that "Now we have PR templates on github" but I can't see any. Same for the main Symfony repository.

[xabbuh]

@mvar At some point we decided that most of the time having to fill out a table is too cumbersome for simple documentation contributions and we wanted to further lower the burden to contribute to the docs.