Revert "Check for AVX512 at runtime to avoid invalid opcode trap (#8166)"

This reverts commit ef5fb6fa46.

core/build.rs

@@ -1,8 +0,0 @@
-fn main() {
-    // Until https://github.com/rust-lang/rust/issues/44839 is resolved
-    // `#[target_feature(enable = "avx512f")]` is not available.
-    // Use an env flag instead:
-    if is_x86_feature_detected!("avx512f") {
-        println!("cargo:rustc-env=TARGET_FEATURE_AVX512F=1");
-    }
-}