Kernel: Use default con/de-structors

This may seem like a no-op change, however it shrinks down the Kernel by a bit:
.text -432
.unmap_after_init -60
.data -480
.debug_info -673
.debug_aranges 8
.debug_ranges -232
.debug_line -558
.debug_str -308
.debug_frame -40

With '= default', the compiler can do more inlining, hence the savings.
I intentionally omitted some opportunities for '= default', because they
would increase the Kernel size.

Kernel/FileSystem/BlockBasedFileSystem.cpp

@@ -51,7 +51,7 @@ public:
         }
     }
 
-    ~DiskCache() { }
+    ~DiskCache() = default;
 
     bool is_dirty() const { return m_dirty; }
     void set_dirty(bool b) { m_dirty = b; }

Kernel/FileSystem/FileDescription.h

@@ -40,7 +40,7 @@ namespace Kernel {
 
 class FileDescriptionData {
 public:
-    virtual ~FileDescriptionData() { }
+    virtual ~FileDescriptionData() = default;
 };
 
 class FileDescription : public RefCounted<FileDescription> {

Kernel/FileSystem/InodeIdentifier.h

@@ -40,7 +40,7 @@ TYPEDEF_DISTINCT_ORDERED_ID(unsigned, InodeIndex);
 
 class InodeIdentifier {
 public:
-    InodeIdentifier() { }
+    InodeIdentifier() = default;
     InodeIdentifier(u32 fsid, InodeIndex inode)
         : m_fsid(fsid)
         , m_index(inode)

Kernel/FileSystem/ProcFS.h

@@ -58,7 +58,7 @@ private:
     ProcFS();
 
     struct ProcFSDirectoryEntry {
-        ProcFSDirectoryEntry() { }
+        ProcFSDirectoryEntry() = default;
         ProcFSDirectoryEntry(const char* a_name, unsigned a_proc_file_type, bool a_supervisor_only, bool (*read_callback)(InodeIdentifier, KBufferBuilder&) = nullptr, ssize_t (*write_callback)(InodeIdentifier, const UserOrKernelBuffer&, size_t) = nullptr, RefPtr<ProcFSInode>&& a_inode = nullptr)
             : name(a_name)
             , proc_file_type(a_proc_file_type)