C#: Use shared default-case handling for C#

Keep only language-specific switch-arm match-all logic, since the shared
control-flow graph now handles default cases directly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4a9b52f8-d07b-4692-8fb9-409f0efd64f2

Author: paldepind

csharp/ql/lib/semmle/code/csharp/controlflow/ControlFlowGraph.qll

@@ -135,7 +135,7 @@ private module Input implements InputSig1, InputSig2 {
 
   predicate catchAll(Ast::CatchClause catch) { catch instanceof GeneralCatchClause }
 
-  predicate matchAll(Ast::Case c) { c instanceof DefaultCase or c.(SwitchCaseExpr).matchesAll() }
+  predicate matchAll(Ast::Case c) { c.(SwitchCaseExpr).matchesAll() }
 
   private newtype TLabel =
     TLblGoto(string label) { any(GotoLabelStmt goto).getLabel() = label } or