Kernel: Stop using *LockRefPtr for FileSystem pointers

There was only one permanent storage location for these: as a member
in the Mount class.

That member is never modified after Mount initialization, so we don't
need to worry about races there.

Kernel/FileSystem/DevPtsFS/FileSystem.cpp

@@ -13,9 +13,9 @@
 
 namespace Kernel {
 
-ErrorOr<NonnullLockRefPtr<FileSystem>> DevPtsFS::try_create()
+ErrorOr<NonnullRefPtr<FileSystem>> DevPtsFS::try_create()
 {
-    return TRY(adopt_nonnull_lock_ref_or_enomem(new (nothrow) DevPtsFS));
+    return TRY(adopt_nonnull_ref_or_enomem(new (nothrow) DevPtsFS));
 }
 
 DevPtsFS::DevPtsFS() = default;