Update PR and Bug templates (#741)

We've already discussed removing "Why is this important?" from the
PR template. I went a step further and removed "Expected behavior"
from the Bug_report template, as I think it's almost always addressed in
the "Describe the bug" section.

Author: basepi

.github/ISSUE_TEMPLATE/Bug_report.md

@@ -13,12 +13,10 @@ about: Create a report to help us improve
 3. ...
 4. ...
 
-**Expected behavior**: ...
-
 **Environment (please complete the following information)**
 - OS: [e.g. Linux]
 - Python version:
-- Framework and version [e.g. Django 2.1]: 
+- Framework and version [e.g. Django 2.1]:
 - APM Server version:
 - Agent version:
 

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,11 +4,5 @@
 Here you can explain the changes made on the PR.
 -->
 
-## Why is it important?
-
-<!-- Comment:
-Here you can explains how this change will impact the funcionality of the agent
--->
-
 ## Related issues
 closes #ISSUE