[misc] remove unused issue template

Signed-off-by: Leo Chen <leo@harmony.one>

.github/ISSUE_TEMPLATE/stake-heist-submission.md

@@ -1,22 +0,0 @@
----
-name: stake heist submission
-about: Submit work for Stake Heist Hackathon
-title: "[submission]"
-labels: stake-heist
-assignees: gizemcakil
-
----
-
-# Linked bounty (please link the related bounty posting for your work)
-
-# Description of the bug, vulnerability, security threat OR the product/tool
-
-# Steps to reproduce the reported vulnerability OR steps taken to build the product/tool
-
-# Proof of exploitability OR proof of use-case (e.g. screenshot, video, github code source)
-
-# Perceived impact to another user or the organization
-
-# List of URLs and affected parameters (if applicable)
-
-# Browser, OS and/or app version used during testing (if applicable)