Remove issue title prefixes

Use labels to categorize them instead.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -2,8 +2,6 @@
 name: Bug report
 about: Doom might be misbehaving
 labels: is:bug
-title: "[BUG] "
-assignees: ''
 ---
 
 **What did you expect to happen?**

.github/ISSUE_TEMPLATE/feature_request.md

@@ -2,8 +2,6 @@
 name: Feature request
 about: It'd be cool if Doom did/had/would...
 labels: is:new
-title: "[REQUEST] "
-assignees: ''
 ---
 
 **Describe the feature**

.github/ISSUE_TEMPLATE/how_to.md

@@ -2,8 +2,6 @@
 name: How do I...
 about: How to get Doom/Emacs to behave a certain way
 labels: is:question
-title: "[HOWTO] "
-assignees: ''
 ---
 
 **What are you trying to achieve?**