RFR: 8311085: Remove implementation detail writeTo from LocalVariable(Type)

[Chen Liang]

`LocalVariable` and `LocalVariableType` includes `writeTo(BufWriter)`, which should be implementation details.

See https://mail.openjdk.org/pipermail/classfile-api-dev/2023-June/000381.html for context.

This patch moves the implementation to `DirectCodeBuilder`'s original use sites; the old `b.canWriteDirect` branch   is redundant, as `writeIndex`'s implementation already performs such an optimization.

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Commit messages:
 - oops, CorpusTest used this internal API
 - Remove implementation detail writeTo from LocalVariable(Type)

Changes: https://git.openjdk.org/jdk/pull/14705/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14705&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311085
  Stats: 90 lines in 6 files changed: 37 ins; 49 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14705.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14705/head:pull/14705

PR: https://git.openjdk.org/jdk/pull/14705