"variable does not need to be mutable" compile warnings in components/script

[Oriol Brufau]

After #31813, when compiling I get

warning: variable does not need to be mutable
  --> components/script/dom/bindings/structuredclone.rs:58:5
   |
58 |     mut sc_holder: &mut StructuredDataHolder,
   |     ----^^^^^^^^^
   |     |
   |     help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

warning: variable does not need to be mutable
   --> components/script/dom/bindings/structuredclone.rs:169:13
    |
169 |         let mut sc_holder = &mut *(closure as *mut StructuredDataHolder);
    |             ----^^^^^^^^^
    |             |
    |             help: remove this `mut`

warning: variable does not need to be mutable
   --> components/script/dom/bindings/structuredclone.rs:197:13
    |
197 |         let mut sc_holder = &mut *(closure as *mut StructuredDataHolder);
    |             ----^^^^^^^^^
    |             |
    |             help: remove this `mut`

warning: variable does not need to be mutable
  --> components/script/dom/crypto.rs:54:17
   |
54 |             let mut data = unsafe { input.as_mut_slice() };
   |                 ----^^^^
   |                 |
   |                 help: remove this `mut`

warning: variable does not need to be mutable
  --> components/script/dom/cssstyledeclaration.rs:75:25
   |
75 |                     let mut pdb = lock.write_with(&mut guard);
   |                         ----^^^
   |                         |
   |                         help: remove this `mut`