[Mono] Remove unused field in Exception.Mono (#71202)

https://github.com/dotnet/runtime/pull/70970 was merged with a field that was unused.  This PR removes it.
Co-authored-by: NSteve Pfister <steve.pfister@microsoft.com>

src/mono/System.Private.CoreLib/src/System/Exception.Mono.cs

@@ -49,8 +49,6 @@ public DispatchState(MonoStackFrame[]? stackFrames)
 
         private bool HasBeenThrown => _traceIPs != null;
 
-        private readonly object frameLock = new object();
-
         public MethodBase? TargetSite
         {
             [RequiresUnreferencedCode("Metadata for the method might be incomplete or removed")]