RFR: 8264484: Replace uses of StringBuffer with StringBuilder in jdk.hotspot.agent [v2]

[Andrey Turbanov]

> Found by IntelliJ IDEA inspection `Java | Java language level migration aids | Java 5 | 'StringBuffer' may be 'StringBuilder'`
> As suggested in https://github.com/openjdk/jdk/pull/1507#issuecomment-757369003 I've created separate PR for module `jdk.hotspot.agent`
> Similar cleanups in the past: https://bugs.openjdk.java.net/browse/JDK-8041679 https://bugs.openjdk.java.net/browse/JDK-8264029

Andrey Turbanov has updated the pull request incrementally with one additional commit since the last revision:

  8264484: Replace uses of StringBuffer with StringBuilder in jdk.hotspot.agent
  
  remove redundant local variables as suggested by reviewers

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3275/files
  - new: https://git.openjdk.java.net/jdk/pull/3275/files/cd4fbf1d..75e4f6b0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3275&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3275&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3275.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3275/head:pull/3275

PR: https://git.openjdk.java.net/jdk/pull/3275