Revert "Add release note blurb about TypeMention.toString()"

This change didn't make into 1.21, so it doesn't belong in this file.

This reverts commit 8c09900.

Author: jbj

change-notes/1.21/analysis-cpp.md

@@ -39,5 +39,4 @@
     - The taint tracking library now includes taint-specific edges for functions modeled in `semmle.code.cpp.models.interfaces.DataFlow`.
     - The taint tracking library adds flow through library functions that are modeled in `semmle.code.cpp.models.interfaces.Taint`. Queries can add subclasses of `TaintFunction` to specify additional flow.
 - There is a new `FoldExpr` class, representing C++17 fold expressions.
-- The member predicates `DeclarationEntry.getUnspecifiedType`, `Expr.getUnspecifiedType`, and `Variable.getUnspecifiedType` have been added. These should be preferred over the existing `getUnderlyingType` predicates.
-- The predicate `TypeMention.toString()` has been simplified to always return the string "`type mention`".  This may improve performance when using `Element.toString()` or its descendants.
+- The member predicates `DeclarationEntry.getUnspecifiedType`, `Expr.getUnspecifiedType`, and `Variable.getUnspecifiedType` have been added. These should be preferred over the existing `getUnderlyingType` predicates.