RFR: 8265688: Unused ciMethodType::ptype_at should be removed

[Damon Fenacci]

`ciMethodType::ptype_at` method is not used.

Removing it.

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Commit messages:
 - JDK-8265688 Unused ciMethodType::ptype_at should be removed

Changes: https://git.openjdk.org/jdk/pull/11708/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11708&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8265688
  Stats: 12 lines in 2 files changed: 0 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11708.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11708/head:pull/11708

PR: https://git.openjdk.org/jdk/pull/11708