Do not specify an SDK version in object files

This is unnecessary, since it ends up being overwritten when linking
anyhow, and it feels wrong to embed some arbitrary SDK version in here.

compiler/rustc_target/src/spec/mod.rs

@@ -61,7 +61,7 @@ pub mod crt_objects;
 mod base;
 pub use base::apple::{
     deployment_target_for_target as current_apple_deployment_target,
-    platform as current_apple_platform, sdk_version as current_apple_sdk_version,
+    platform as current_apple_platform,
 };
 pub use base::avr_gnu::ef_avr_arch;