mcp: use blank issue title

To hopefully make it less likely for people to forget to change their
MCP issue title.

Author: jieyouxu

.github/ISSUE_TEMPLATE/major_change.md

@@ -1,7 +1,7 @@
 ---
 name: Major change proposal (MCP)
 about: Propose a major change.
-title: "(My major change proposal)"
+title: ''
 labels: major-change, T-compiler
 assignees: ''