Use blank title for ACP issue template

There's a near identical PR for compiler MCPs: rust-lang/compiler-team#879

> To hopefully make it less likely for people to forget to change their MCP issue title.
>
> See #t-compiler/major changes > Retroactive MCP for the Rust for Linux Ec… compiler-team#874 @ 💬

Author: programmerjake

.github/ISSUE_TEMPLATE/api-change-proposal.md

@@ -1,7 +1,7 @@
 ---
 name: API Change Proposal
 about: Propose a new API change to the libs-api team
-title: "(My API Change Proposal)"
+title: ''
 labels: api-change-proposal, T-libs-api
 assignees: ""
 ---