RFR: 8283365: G1: Remove duplicate assertions in HeapRegion::oops_on_memregion_seq_iterate_careful

[Albert Mingkun Yang]

Simple change of removing duplicate assertions.

Test: build

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Commit messages:
 - g1-remove-assert

Changes: https://git.openjdk.java.net/jdk/pull/7866/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7866&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283365
  Stats: 10 lines in 1 file changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7866.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7866/head:pull/7866

PR: https://git.openjdk.java.net/jdk/pull/7866