Remove support for alias -Z instrument-coverage

This flag was stabilized in rustc 1.60.0 as `-C instrument-coverage`, but the
old unstable flag was kept around as an alias to ease migration.

compiler/rustc_interface/src/tests.rs

@@ -789,7 +789,6 @@ fn test_unstable_options_tracking_hash() {
     tracked!(inline_mir, Some(true));
     tracked!(inline_mir_hint_threshold, Some(123));
     tracked!(inline_mir_threshold, Some(123));
-    tracked!(instrument_coverage, Some(InstrumentCoverage::All));
     tracked!(instrument_mcount, true);
     tracked!(instrument_xray, Some(InstrumentXRay::default()));
     tracked!(link_directives, false);