Remove Rust workaround for proc-macro2
-removes workaround for can't find crate for proc_macro when compiling proc-macro2` -https://github.com/meta-rust/meta-rust/issues/266 -https://github.com/meta-flutter/meta-flutter/issues/370
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-removes workaround for can't find crate for proc_macro when compiling proc-macro2` -https://github.com/meta-rust/meta-rust/issues/266 -https://github.com/meta-flutter/meta-flutter/issues/370