RFR: 8218738: Remove dead code in Symbol and friends

[Claes Redestad]

Hi,

JDK-8218675 is removing the last use of Symbol::as_utf8_flexible_buffer.
This patch removes it, along with a few other methods, declarations etc
in the area which are also no longer in use.

Webrev: http://cr.openjdk.java.net/~redestad/8218738/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8218738

Testing: tier1

Thanks!

/Claes