Revert "Set MACOSX_DEPLOYMENT_TARGET env var to default for linking if not set."

This reverts commit b376f56, which is
the main part of #90499, because it turns out that this causes a good
amount of breakage in crates relying on the old behavior.

Fixes #91372.

Author: hkratz

compiler/rustc_target/src/spec/aarch64_apple_darwin.rs

@@ -9,7 +9,6 @@ pub fn target() -> Target {
     base.supported_sanitizers = SanitizerSet::ADDRESS | SanitizerSet::CFI | SanitizerSet::THREAD;
 
     base.pre_link_args.insert(LinkerFlavor::Gcc, vec!["-arch".to_string(), "arm64".to_string()]);
-    base.link_env.extend(super::apple_base::macos_link_env("arm64"));
     base.link_env_remove.extend(super::apple_base::macos_link_env_remove());
 
     // Clang automatically chooses a more specific target based on

compiler/rustc_target/src/spec/apple_base.rs

@@ -79,18 +79,6 @@ pub fn macos_llvm_target(arch: &str) -> String {
     format!("{}-apple-macosx{}.{}.0", arch, major, minor)
 }
 
-pub fn macos_link_env(arch: &str) -> Vec<(String, String)> {
-    // Use the default deployment target for linking just as with the LLVM target if not
-    // specified via MACOSX_DEPLOYMENT_TARGET, otherwise the system linker would use its
-    // default which varies with Xcode version.
-    if env::var("MACOSX_DEPLOYMENT_TARGET").is_err() {
-        let default = macos_default_deployment_target(arch);
-        vec![("MACOSX_DEPLOYMENT_TARGET".to_string(), format!("{}.{}", default.0, default.1))]
-    } else {
-        vec![]
-    }
-}
-
 pub fn macos_link_env_remove() -> Vec<String> {
     let mut env_remove = Vec::with_capacity(2);
     // Remove the `SDKROOT` environment variable if it's clearly set for the wrong platform, which

compiler/rustc_target/src/spec/i686_apple_darwin.rs

@@ -5,7 +5,6 @@ pub fn target() -> Target {
     base.cpu = "yonah".to_string();
     base.max_atomic_width = Some(64);
     base.pre_link_args.insert(LinkerFlavor::Gcc, vec!["-m32".to_string()]);
-    base.link_env.extend(super::apple_base::macos_link_env("i686"));
     base.link_env_remove.extend(super::apple_base::macos_link_env_remove());
     // don't use probe-stack=inline-asm until rust#83139 and rust#84667 are resolved
     base.stack_probes = StackProbeType::Call;

compiler/rustc_target/src/spec/x86_64_apple_darwin.rs

@@ -10,7 +10,6 @@ pub fn target() -> Target {
         LinkerFlavor::Gcc,
         vec!["-m64".to_string(), "-arch".to_string(), "x86_64".to_string()],
     );
-    base.link_env.extend(super::apple_base::macos_link_env("x86_64"));
     base.link_env_remove.extend(super::apple_base::macos_link_env_remove());
     // don't use probe-stack=inline-asm until rust#83139 and rust#84667 are resolved
     base.stack_probes = StackProbeType::Call;