Drop a few unnecessary no_mangle

[Pantelis Sopasakis]

We cannot have #[no_mangle] on structs:

Warning:   --> src/lib.rs:67:1
   |
67 |   #[no_mangle]
   |   ^^^^^^^^^^^^
68 | / pub struct rosenbrock_rosCache {
69 | |     cache: AlmCache,
70 | | }
   | |_- not a function or static
   |
   = note: `#[warn(unused_attributes)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!

This needs to be changed in templates/c/optimizer_cinterface.rs.