Rust: Remove obsolete bindgen flag

--size_t-is-usize has been deprecated for a while and is
removed in bindgen 0.64

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>

build-aux/rust-regen.sh

@@ -17,7 +17,6 @@ filter="$3"
 shift; shift; shift
 
 bindgen \
-	--size_t-is-usize \
 	--no-prepend-enum-name \
 	--no-layout-tests \
 	--no-doc-comments \