MAINTAINERS: Remove Alex Gaynor as Rust maintainer

I've long since stopped having the time to contribute code or reviews,
this acknowledges that.

Geoffrey Thomas and I created the "linux-kernel-module-rust" project at
PyCon in 2018, as an experiment to see if we could make it possible to
write kernel modules in Rust. The Rust for Linux effort has far exceeded
anything we could have expected at the time.

I want to thank all the Rust for Linux contributors, past and present,
who have helped make this a reality -- and in particularly Miguel, who
really transformed this project from an interesting demo to something
that could really land in mainline.

Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Link: https://patch.msgid.link/CAFRnB2U1uvg1vyZe1kDi7L3P4kTFowfOo6Hfo9WJED4qve4ZZw@mail.gmail.com
[ Reflowed. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>