RFR: 8217383: Obsolete UseImplicitStableValues

[Claes Redestad]

Hi,

the diagnostic flag UseImplicitStableValues has no effect since
String.value was made explicitly @Stable in JDK 9. This patch obsoletes
the flag and removes the special-casing of String.value.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8217383
Webrev: http://cr.openjdk.java.net/~redestad/8217383/open.00/

Testing: tier1-3, verified no regressions in a few String
microbenchmarks

CSR filed and approved.

Thanks!

/Claes