Remove msrv field from .clippy.toml

Since Rust 1.64, Clippy respects `rust-version` field in Cargo.toml.
rust-lang/rust@b776fb8

Author: taiki-e

.clippy.toml

@@ -40,7 +40,7 @@ jobs:
     strategy:
       matrix:
         # When updating this, the reminder to update the minimum supported
-        # Rust version in Cargo.toml and .clippy.toml.
+        # Rust version in Cargo.toml.
         rust: ['1.39']
     steps:
       - uses: actions/checkout@v3