[vfs] Remove transaction counting

This functionality is only used to implement assertions in driver_host;
it isn't clear why these assertions are useful.

This functionality is tested using fs::Vnode::HandleFsSpecificMessage
which we'd like to remove.

Change-Id: I2d93d6fee184c3fc776aaa2f82d0c6170ce55ed9
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/767384
Fuchsia-Auto-Submit: Tamir Duberstein <tamird@google.com>
Reviewed-by: David Gilhooley <dgilhooley@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>