RFR(T) 8217578: Shenandoah cleanup unused timings after concurrent string table change

[zgu at redhat.com]

Oops in string table were moved into OopStorage, oop iterations were
also folded into WeakProcessor. There is no more separate phases for
string table in Shenandoah code, let's remove obsoleted timings.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217578
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8217578/webrev.00/

Test:
  tier1, 2, 3_gc_shenandoah on Linux x64 (fastdebug and release)
  Eyeball -Xlog:gc+stats output

Thanks,

-Zhengyu