Remove unused mut

[MilesCranmer]

FYI I'm not sure if this is correct or not, but cargo told me to submit this fix:

warning: variable does not need to be mutable
   --> src/metrics/mod.rs:663:17
    |
663 | ...   let mut zd = self.disks.entry(name).or_insert(ZDisk::fro...
    |           ----^^
    |           |
    |           help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` on by default

warning: variable does not need to be mutable
   --> src/metrics/mod.rs:728:13
    |
728 |         let mut overall = self
    |             ----^^^^^^^
    |             |
    |             help: remove this `mut`

[bvaisvil]

Thanks for the PR. I think it's probably right. The reference does not need to be mutable.