[foreign-memaccess+abi] RFR: Clean up StringSupport

[Per Minborg]

This PR suggests some minor cleanups of the class `StringSupport`.

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Commit messages:
 - Clean up StringSupport

Changes: https://git.openjdk.org/panama-foreign/pull/863/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=863&range=00
  Stats: 5 lines in 1 file changed: 2 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/863.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/863/head:pull/863

PR: https://git.openjdk.org/panama-foreign/pull/863