RFR: 8245455: Remove alternative StringConcatFactory strategies

[Claes Redestad]

Hi,

this patch removes the alternative, undocumented strategies from
StringConcatFactory.

The default strategy has been optimized and stabilized since inception 
in JDK 9, while the alternative strategies have been falling behind
both in terms of performance and stability - while adding maintenance
cost.

The removal allows some simplifications to the code that will reduce
per callsite bootstrap overhead, and reduce what we need to load eagerly
during bootstrap. I've also applied the pattern from JDK-8218173 to
the remaining Stringifier method handles, which further improve
robustness of bootstrapping under an OOME condition.

Webrev: http://cr.openjdk.java.net/~redestad/8245455/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8245455

Testing: tier1-3

Thanks!

/Claes