RFR: Simplify code to use `withoutName`

[Maurizio Cimadamore]

This patch simplifies some recently added code to just use the newly added layout API method to drop a name from a value layout.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/112/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=112&range=00
  Stats: 29 lines in 1 file changed: 0 ins; 27 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/112.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/112/head:pull/112

PR: https://git.openjdk.org/jextract/pull/112