doc: Remove label from good first issue template

Author: MarcoFalke

.github/ISSUE_TEMPLATE/good_first_issue.md

@@ -2,11 +2,13 @@
 name: Good first issue
 about: '(Regular devs only): Suggest a new good first issue'
 title: ''
-labels: good first issue
+labels: ''
 assignees: ''
 
 ---
 
+<!-- Needs the label "good first issue" assigned manually before or after opening -->
+
 <!-- A good first issue is an uncontroversial issue, that has a relatively unique and obvious solution -->
 
 <!-- Motivate the issue and explain the solution briefly -->