coverage: Remove confusing comments from CoverageKind

These comments appear to be inspired by the similar comments on
`CounterIncrement` and `ExpressionUsed`. But those comments refer to specific
simplification steps performed during coverage codegen, and there is no
corresponding step for the MC/DC coverage statements.

If these statements do not survive optimization, they will simply not
participate in code generation, just like any other statement.

compiler/rustc_middle/src/mir/coverage.rs

@@ -129,17 +129,11 @@ pub enum CoverageKind {
     /// Marks the point in MIR control flow represented by a evaluated condition.
     ///
     /// This is eventually lowered to `llvm.instrprof.mcdc.condbitmap.update` in LLVM IR.
-    ///
-    /// If this statement does not survive MIR optimizations, the condition would never be
-    /// taken as evaluated.
     CondBitmapUpdate { id: ConditionId, value: bool, decision_depth: u16 },
 
     /// Marks the point in MIR control flow represented by a evaluated decision.
     ///
     /// This is eventually lowered to `llvm.instrprof.mcdc.tvbitmap.update` in LLVM IR.
-    ///
-    /// If this statement does not survive MIR optimizations, the decision would never be
-    /// taken as evaluated.
     TestVectorBitmapUpdate { bitmap_idx: u32, decision_depth: u16 },
 }