Remove PR size notifications (#52373)

These are noisy, and in general I'd prefer people to focus on the
quality of the resulting system; not the size of the diff. (Which may
require deliberately making changes larger)

## Context

<!-- What does this PR do, and why? How is it expected to impact users?
     Not just what changed, but what motivated it and why this approach.

Link to Linear issue (e.g., ENG-123) or GitHub issue (e.g., Closes #456)
     if one exists — helps with traceability. -->

## How to Review

<!-- Help reviewers focus their attention:
- For small PRs: note what to focus on (e.g., "error handling in
foo.rs")
- For large PRs (>400 LOC): provide a guided tour — numbered list of
files/commits to read in order. (The `large-pr` label is applied
automatically.)
     - See the review process guidelines for comment conventions -->

## Self-Review Checklist

<!-- Check before requesting review: -->
- [ ] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Release Notes:

- N/A

.github/pull_request_template.md

@@ -1,28 +1,13 @@
-## Context
+Self-Review Checklist:
 
-<!-- What does this PR do, and why? How is it expected to impact users?
-     Not just what changed, but what motivated it and why this approach.
-
-     Link to Linear issue (e.g., ENG-123) or GitHub issue (e.g., Closes #456)
-     if one exists — helps with traceability. -->
-
-## How to Review
-
-<!-- Help reviewers focus their attention:
-     - For small PRs: note what to focus on (e.g., "error handling in foo.rs")
-     - For large PRs (>400 LOC): provide a guided tour — numbered list of
-       files/commits to read in order. (The `large-pr` label is applied automatically.)
-     - See the review process guidelines for comment conventions -->
-
-## Self-Review Checklist
-
-<!-- Check before requesting review: -->
 - [ ] I've reviewed my own diff for quality, security, and reliability
 - [ ] Unsafe blocks (if any) have justifying comments
 - [ ] The content is consistent with the [UI/UX checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
 - [ ] Tests cover the new/changed behavior
 - [ ] Performance impact has been considered and is acceptable
 
+Closes #ISSUE
+
 Release Notes:
 
 - N/A or Added/Fixed/Improved ...