Remove _storedCompressedFabricID on MTRDeviceController.

Both subclasses override the getter, so we should just mark it as abstract and
stop pretending to have state we don't actually have.

src/darwin/Framework/CHIP/MTRDeviceController.mm

@@ -53,7 +53,6 @@
 
 #include <atomic>
 #include <dns_sd.h>
-#include <optional>
 #include <string>
 
 #import <os/lock.h>
@@ -94,8 +93,6 @@ - (instancetype)initWithDelegate:(id<MTRDeviceControllerDelegate>)delegate queue
 @implementation MTRDeviceController {
     os_unfair_lock _underlyingDeviceMapLock;
 
-    std::atomic<std::optional<uint64_t>> _storedCompressedFabricID;
-
     // For now, we just ensure that access to _suspended is atomic, but don't
     // guarantee atomicity of the the entire suspend/resume operation.  The
     // expectation is that suspend/resume on a given controller happen on some
@@ -504,8 +501,8 @@ - (void)asyncDispatchToMatterQueue:(dispatch_block_t)block errorHandler:(nullabl
 
 - (nullable NSNumber *)compressedFabricID
 {
-    auto storedValue = _storedCompressedFabricID.load();
-    return storedValue.has_value() ? @(storedValue.value()) : nil;
+    MTR_ABSTRACT_METHOD();
+    return nil;
 }
 
 #ifdef DEBUG