RFR: 8254824: SignalHandlerMark have no purpose

[Robbin Ehn]

SignalHandlerMark increase and decrease _num_nested_signal, but it is never read.
We have no caller to is_inside_signal_handler().

Testing T1.

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Commit messages:
 - SignalHandlerMark removed

Changes: https://git.openjdk.java.net/jdk/pull/677/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=677&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254824
  Stats: 49 lines in 11 files changed: 0 ins; 49 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/677.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/677/head:pull/677

PR: https://git.openjdk.java.net/jdk/pull/677