This is a general cleanup after removing LockingMode related code.
It includes:
- Removing asserts() that are no longer necessary, since we removed legacy locking and monitor locking.
- Rewriting or deleting comments and arguments that are related to displaced headers.
- Etc., etc.

In the review of JDK-8365190 a number of suggestions for further clean up came up. I'll list them here so we don't lose them:
1. https://github.com/openjdk/jdk/pull/27041#discussion_r2317063086 (Delete displaced_header_offset_in_bytes)
2. https://github.com/openjdk/jdk/pull/27041#discussion_r2317069783 (Can we now remove _held_monitor_count next)
3. https://github.com/openjdk/jdk/pull/27041#discussion_r2317095107 (Remove dispatch functions)
4. https://github.com/openjdk/jdk/pull/27041#discussion_r2318041555 (Simplify and unify this with FastHashCode)
5. https://github.com/openjdk/jdk/pull/27041#discussion_r2317998409 (Remove parameter from MonitorExitStub)
6. https://github.com/openjdk/jdk/pull/27041#discussion_r2320634417 (Re-type/name metadata in BasicLock)
7. https://github.com/openjdk/jdk/pull/27041#discussion_r2320643671 (Delete displaced_header_offset_in_bytes)
8. https://github.com/openjdk/jdk/pull/27041#discussion_r2317088830 (Eliminating the middle call)

Number 1, 5, 6 and 7 will be taken care of in this RFE.
Number 2 will be taken care of in JDK-8367601.
Number 3, 4 and 8 will be taken care of in JDK-8367982.