fix(build-rs)!: Remove meaningless 'cargo_cfg_debug_assertions'

The documentation that was added was pulled straight from a comment in
`custom_build.rs`.

crates/build-rs-test-lib/build.rs

@@ -10,7 +10,6 @@ fn smoke_test_inputs() {
     use build_rs::input::*;
     dbg!(cargo());
     dbg!(cargo_cfg("careful"));
-    dbg!(cargo_cfg_debug_assertions());
     #[cfg(feature = "unstable")]
     dbg!(cargo_cfg_fmt_debug());
     #[cfg(feature = "unstable")]

crates/build-rs/src/input.rs

@@ -109,6 +109,10 @@ mod cfg {
     }
 
     /// If we are compiling with debug assertions enabled.
+    ///
+    /// This cfg is always true and misleading, so avoid setting it.
+    /// That is because Cargo queries rustc without any profile settings.
+    #[cfg(any())]
     #[track_caller]
     pub fn cargo_cfg_debug_assertions() -> bool {
         is_present("CARGO_CFG_DEBUG_ASSERTIONS")