RFR: 8221219: ZGC: Remove ZStallOnOutOfMemory option

[Per Liden]

The ZStallOnOutOfMemory option was added in ZGC's early days, and was at 
that time useful when testing and debugging. These days it's not so 
useful, and I can't really see anyone wanting to use it when running a 
real application as is can/will cause pre-mature OOMEs to be thrown. I 
suggest we remove it.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221219
Webrev: http://cr.openjdk.java.net/~pliden/8221219/webrev.0

(Change builds on JDK-8221212, which is out for review)

/Per