Remove unused RUSTC_DEBUG_ASSERTIONS
Remove unused RUSTC_DEBUG_ASSERTIONS

Since #73374 the rustc wrapper no longer configures debug assertions
based on RUSTC_DEBUG_ASSERTIONS environment variable.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust