tests: Remove rust-toolchain.toml from AVR test

```
error[E0658]: use of unstable library feature 'proc_macro_byte_character'
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.80/src/wrapper.rs:871:21
    |
871 |                     proc_macro::Literal::byte_character(byte)
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #115268 <rust-lang/rust#115268> for more information
    = help: add `#![feature(proc_macro_byte_character)]` to the crate attributes to enable
    = note: this compiler was built on 2024-03-21; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'proc_macro_c_str_literals'
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.80/src/wrapper.rs:898:21
    |
898 |                     proc_macro::Literal::c_string(string)
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #119750 <rust-lang/rust#119750> for more information
    = help: add `#![feature(proc_macro_c_str_literals)]` to the crate attributes to enable
    = note: this compiler was built on 2024-03-21; consider upgrading it if it is out of date
```

Author: taiki-e

tests/avr/Cargo.toml

@@ -12,8 +12,6 @@ paste = "1"
 ufmt = "0.2"
 
 [target.avr-unknown-gnu-atmega2560.dependencies]
-# When updating the revision, the reminder to update rust-toolchain.toml to use
-# the same toolchain as https://github.com/Rahix/avr-hal/blob/HEAD/rust-toolchain.toml.
 arduino-hal = { git = "https://github.com/taiki-e/avr-hal.git", branch = "dev", features = ["arduino-mega2560"] }
 
 [workspace]