Remove description field in PR template (#583)

## Type of Change
- [x] Process update

## Description
Remove description field in PR template 

## Motivation and Context
I think having separate `Description` and `Motivation and Context`
sections have been redundant in recent usage of the PR template. This PR
removes the `Description` section. Ideally the PR title should cover the
same content previously in the `Description` section.

Signed-off-by: Kathryn Baldauf <k_baldauf@apple.com>

Author: katiewasnothere

.github/pull_request_template.md

@@ -4,14 +4,10 @@
 - [ ] Breaking change
 - [ ] Documentation update
 
-## Description
-[Brief description]
-
 ## Motivation and Context
 [Why is this change needed?]
 
 ## Testing
 - [ ] Tested locally
 - [ ] Added/updated tests
 - [ ] Added/updated docs
-