RFR: 8276229: Stop allowing implicit updates in G1BlockOffsetTable

[Stefan Johansson]

Please review this change that removes some code that is no longer needed.

**Summary**
In [JDK-8272083](https://bugs.openjdk.java.net/browse/JDK-8276229) G1 was changed to create a precise and complete BOT during evacuation (in the GC pause). This removes the need to update the BOT when looking up block starts. This previously happened in concurrent refinement and while scanning the heap in the pause, but with the precise BOT we should no longer require any such updates.

This change removes the slow path function that did the updates `forward_to_block_containing_addr_slow(...)`. It also renames some functions to no longer include a `_const` post-fix because there are only "const" versions of the functions remaining.

**Testing**

- [x] Mach5 1-3
- [x] Local stress-testing

-------------

Commit messages:
 - 8276229: Stop allowing implicit updates in G1BlockOffsetTable

Changes: https://git.openjdk.java.net/jdk/pull/6332/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6332&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276229
  Stats: 95 lines in 4 files changed: 8 ins; 81 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6332.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6332/head:pull/6332

PR: https://git.openjdk.java.net/jdk/pull/6332