autoload/rustfmt.vim should not call rustfmt on load

[cjhopman]

Even with rustfmt disabled, we're going to load autoload/rustfmt.vim

Currently, this will call rustfmt --help and rustfmt --version when loading. This can rarely take an extremely long time (like when cargo needs to download new toolchain components).

Encountering this really slow load on some interactions causes vim to have pretty bad behavior (for example, when a source control tool opens the editor for resolving merge conflict, encountering this slow load when closing the editor feels completely broken).