coverage: Remove all unstable support for MC/DC instrumentation

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -296,10 +296,6 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx, consts_debug_info.unwrap_or_default());
 
-    // If the backend supports coverage, and coverage is enabled for this function,
-    // do any necessary start-of-function codegen (e.g. locals for MC/DC bitmaps).
-    start_bx.init_coverage(instance);
-
     // The builders will be created separately for each basic block at `codegen_block`.
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);

compiler/rustc_codegen_ssa/src/traits/coverageinfo.rs

@@ -2,11 +2,6 @@ use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::ty::Instance;
 
 pub trait CoverageInfoBuilderMethods<'tcx> {
-    /// Performs any start-of-function codegen needed for coverage instrumentation.
-    ///
-    /// Can be a no-op in backends that don't support coverage instrumentation.
-    fn init_coverage(&mut self, _instance: Instance<'tcx>) {}
-
     /// Handle the MIR coverage info in a backend-specific way.
     ///
     /// This can potentially be a no-op in backends that don't support