RFR: 8301148: Serial: Remove ContiguousSpace::reset_saved_mark

[Albert Mingkun Yang]

Simple change of removing redundant code.

Test: hotspot_gc

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Commit messages:
 - s1-remove-reset_saved_mark

Changes: https://git.openjdk.org/jdk/pull/12215/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12215&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301148
  Stats: 2 lines in 2 files changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12215.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12215/head:pull/12215

PR: https://git.openjdk.org/jdk/pull/12215