remove redundant fields from issue template

Author: jenshu

.github/ISSUE_TEMPLATE/enhancement.md

@@ -11,12 +11,6 @@ assignees:
 - One-line enhancement description (can be used as a release note):
 - Kubernetes Enhancement Proposal: <!-- link to kubernetes/enhancements file; if none yet, link to PR -->
 - Discussion Link: <!-- link to SIG mailing list thread, meeting, or recording where the Enhancement was discussed before KEP creation -->
-- Primary contact (assignee):
-- Responsible SIGs:
-- Enhancement target (which target equals to which milestone):
-  - Alpha release target (x.y):
-  - Beta release target (x.y):
-  - Stable release target (x.y):
 - [ ] Alpha
   - [ ] KEP (`k/enhancements`) update PR(s):
   - [ ] Code (`k/k`) update PR(s):