Remove unnecessary mutability

[jstasiak]

This was generating warnings:

warning: variable does not need to be mutable
  --> src/codec/codec007.rs:86:13
   |
86 |         let mut row = &mut dst.buf[start..end];
   |             ----^^^
   |             |
   |             help: remove this `mut`
   |
   = note: #[warn(unused_mut)] on by default

warning: variable does not need to be mutable
  --> src/codec/codec015.rs:50:13
   |
50 |         let mut row = &mut row[start..end];
   |             ----^^^
   |             |
   |             help: remove this `mut`