RFR: 8237508: Simplify JarFile.isInitializing

[Claes Redestad]

Hi,

JDK-8234466[1] introduced isInitializing to JarFile, which cause a small
startup regression in various tests due increasing the number of classes
loaded and earlier lambda bootstrapping. The regression can be resolved
by not explicitly initializing the thread local variable to a non-null
value.

Webrev: http://cr.openjdk.java.net/~redestad/8237508/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8237508

Testing: tier1-3

Thanks!

/Claes

[1] https://bugs.openjdk.java.net/browse/JDK-8234466