Remove unused manifest key for min Rust version

jeffparsons · jeffparsons · commit bfcf48c1efc8 · 2020-07-13T07:18:27.000+10:00
RFC 2495 (rust-lang/rfcs#2495) was merged quite a while ago, but I think it was never actually implemented.
diff --git a/Cargo.toml b/Cargo.toml
@@ -1,7 +1,6 @@
 [package]
 name = "rangemap"
 version = "0.1.5"
-rust = "1.43"
 authors = ["Jeff Parsons <jeff@parsons.io>"]
 edition = "2018"
 license = "MIT/Apache-2.0"
