(urgent) (15) RFR (trivial): 8235965: Tests using ThreadLocalHandshakes need to stop using it as it no longer exists

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8235965
webrev: http://cr.openjdk.java.net/~dholmes/8235965/webrev/

The ThreadLocalHandshakes flag was made obsolete in JDK 14 and has now 
expired in JDK 15. Tests using it should have been updated when it was 
made obsolete.

test/hotspot/jtreg/runtime/NMT/SafepointPollingPages.java

Had two @run lines with the flag enabled/disabled, so this reduces to 
one @run line without the flag.

  test/hotspot/jtreg/runtime/handshake/HandshakeWalkStackFallbackTest.java

runs the HandshakeWalkStackTest.java test with the flag is disabled. As 
the flag no longer exists and cannot be disabled there is nothing to 
test and this test is removed as it is now exactly the same as 
HandshakeWalkStackTest.

The latter is causing tier1 test failures so I'd like to fix ASAP.

Thanks,
David