RFR 8236625 Remove writeable macro from JVM flags declaration

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8236625
http://cr.openjdk.java.net/~iklam/jdk15/8236625-remove-writeable-flag-macro.v01/

The "writeable" macro was used by commercial JVM flags, which have been
removed from the JDK (see JDK-8205505). No flags in the JDK today use this
macro. It should be removed.

I also inlined some accessor functions in the JVMFlag class for slightly
more compact code.

Thanks
- Ioi