Remove unnecessary flag from rustfmt invocation
Remove unnecessary flag from rustfmt invocation

From rustfmt docs, the --write-mode flag is unecessary when passing the
flag to stdin. If we leave it there, rustfmt-nightly complains that it
cannot find the file named --write-mode, so let's remove the flag.

emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen