RFR: 8271896: Remove unnecessary top address checks in BOT

[Albert Mingkun Yang]

Simple change of removing unnecessary top checks. I left the one `HeapRegion::block_size` as an assert, since it's outside BOT.

Test: hotspot_gc

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Commit messages:
 - bot-top

Changes: https://git.openjdk.java.net/jdk/pull/5013/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5013&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271896
  Stats: 5 lines in 3 files changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5013.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5013/head:pull/5013

PR: https://git.openjdk.java.net/jdk/pull/5013