feat: remove proc-macro dependency

Cargo.toml

@@ -16,6 +16,4 @@ features = ["atmega32u4"]
 
 # The latest releases of `proc-macro2` do not support the rust toolchain that
 # we use.  Thus, we must fix this dependency to an older version where our
-# toolchain is still supported.  See https://github.com/Rahix/avr-hal/issues/537
-# [build-dependencies.proc-macro2]
-# version = "=1.0.79"
+# toolchain is still supported.  See https://github.com/Rahix/avr-hal/issues/537

rust-toolchain.toml

@@ -1,4 +1,4 @@
 [toolchain]
 channel = "nightly-2024-06-13"
 components = [ "rust-src" ]
-profile = "default"
+profile = "complete"