Kernel: Remove declarations for non-existent methods

Author: https://github.com/AtkinsSJ
Commit: 3cbc0fdbb0
Pull-request: https://github.com/SerenityOS/serenity/pull/17203
Reviewed-by: https://github.com/linusg ✅

Kernel/FileSystem/BlockBasedFileSystem.h

@@ -44,7 +44,6 @@ protected:
     void remove_disk_cache_before_last_unmount();
 
 private:
-    DiskCache& cache() const;
     void flush_specific_block_if_needed(BlockIndex index);
 
     mutable MutexProtected<OwnPtr<DiskCache>> m_cache;