Delete GSOC.md

GSoC 2022 is now complete - we can re-add this file later if needed for next year.

.github/ISSUE_TEMPLATE/GSOC.md

@@ -1,17 +0,0 @@
----
-name: 'Google Summer of Code'
-about: See https://github.com/wagtail/wagtail/wiki/Google-Summer-of-Code-2022
-title: ''
-labels: 'GSOC'
-assignees: thibaudcolas, dawnwages
----
-
-## Summary
-
-## Implementation
-
-## Skills
-
-## Mentors
-
-## Aims