RFR: 8297487: G1 Remark: no need to keep alive oop constants of nmethods on stack

[Richard Reingruber]

This pr removes the stackwalks to keep alive oops of nmethods found on stack during G1 remark as it seems redundant. The oops are already kept alive by the [nmethod entry barrier](https://github.com/openjdk/jdk/blob/f26bd4e0e8b68de297a9ff93526cd7fac8668320/src/hotspot/share/gc/shared/barrierSetNMethod.cpp#L85)

Additionally it fixes a comment that says nmethod entry barriers are needed to deal with continuations which, afaik, is not the case. Please correct me and explain if I'm mistaken.

Testing: the patch is included in our daily CI testing since a week. That is most JCK and JTREG tests, also in Xcomp mode, Renaissance benchmark and SAP specific tests with fastdebug and release builds on the standard platforms plus PPC64. There was no failure I could attribute to this change.

I tried to find a jtreg test that is sensitive to the keep alive by omitting it in the nmethod entry barrier and also in G1 remark but without success.

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Commit messages:
 - G1 Remark: no need to keep alive const oops for nmethods on stack

Changes: https://git.openjdk.org/jdk/pull/11314/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11314&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297487
  Stats: 16 lines in 2 files changed: 1 ins; 13 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11314.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11314/head:pull/11314

PR: https://git.openjdk.org/jdk/pull/11314