RFR: 8282096: G1: Remove redundant checks in G1CardSet::free_mem_object

[Albert Mingkun Yang]

Simple change of removing unnecessary checks.

Test: hotspot_gc

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

Commit messages:
 - g1-cardset

Changes: https://git.openjdk.java.net/jdk/pull/7532/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7532&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282096
  Stats: 9 lines in 1 file changed: 0 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7532.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7532/head:pull/7532

PR: https://git.openjdk.java.net/jdk/pull/7532