RFR: 8295014: Remove unnecessary explicit casts to void* in CHeapObjBase

[Kim Barrett]

Please review this change to CHeapObjBase that removes unnecessary explicit
casts from char* to void*.

Testing: mach5 tier1

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Commit messages:
 - remove unnecessary casts

Changes: https://git.openjdk.org/jdk/pull/10623/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10623&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295014
  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/10623.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10623/head:pull/10623

PR: https://git.openjdk.org/jdk/pull/10623