Remove reference to NO ENTRY from the pull request template
Remove reference to NO ENTRY from the pull request template

It now mentioned only in the guide on the wiki. This is to make it less likely
for people to wrote "NO ENTRY" on pull requests that need a release notes
entry.

Fixes sympy/sympy-bot#30.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy