RFR: 8326370: Remove redundant and misplaced micros from StringBuffers

[Claes Redestad]

Some microbenchmarks in org.openjdk.bench.java.lang.StringBuffers seem out-of-place (not testing `StringBuffer`), redundant (covered by other tests like StringSubstring or the various String concatenation tests), or both. This cleans them out. 

My apologies to Sigurd.

-------------

Commit messages:
 - 8326370: Remove redundant and misplaced micros from StringBuffers

Changes: https://git.openjdk.org/jdk/pull/17937/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17937&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326370
  Stats: 38 lines in 1 file changed: 0 ins; 38 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17937.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17937/head:pull/17937

PR: https://git.openjdk.org/jdk/pull/17937