[foreign-preview] RFR: 8283863: Drop support for Constable in MemoryLayout and FunctionDescriptor

[Maurizio Cimadamore]

This patch drops support for `Constable` for `MemoryLayout` and `FunctionDescriptor`, as described in [1].

[1] - https://mail.openjdk.java.net/pipermail/panama-dev/2022-March/016631.html

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Commit messages:
 - Drop constable support and associated tests

Changes: https://git.openjdk.java.net/panama-foreign/pull/672/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=672&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283863
  Stats: 374 lines in 9 files changed: 0 ins; 369 del; 5 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/672.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/672/head:pull/672

PR: https://git.openjdk.java.net/panama-foreign/pull/672