RFR: 8260048: Shenandoah: ShenandoahMarkingContext asserts are unnecessary

[Aleksey Shipilev]

There are two `shenandoah_assert_not_forwarded` asserts that are not necessary in `mark_{strong,weak}`, because the only caller [already asserts](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shenandoah/shenandoahMark.inline.hpp#L272) this higher-level invariant. There is no need to check it in `ShenandoahMarkingContext` once again. This simplifies the fastpath in fastdebug builds.

Additional testing:
 - [x] `hotspot_gc_shenandoah`
 - [x] `tier1`, `tier2` with Shenandoah

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

Commit messages:
 - Drop a few more parentheses
 - 8260048: Shenandoah: ShenandoahMarkingContext asserts are unnecessary

Changes: https://git.openjdk.java.net/jdk/pull/2164/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2164&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260048
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2164.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2164/head:pull/2164

PR: https://git.openjdk.java.net/jdk/pull/2164