[foreign-memaccess+abi] RFR: 8292957: Remove redundant MemoryLayout::is* methods

[Per Minborg]

This PR removes the public API methods `MemoryLayout::isPadding`, `GroupLayout::isStruct`, and `GroupLayout::isSequence`. Now that the `MemoryLayout` sealed hierarchy provides the same information, these redundant methods should be removed for reasons of consistency, symmetry, brevity, and simplicity.

See the details and proposed replacements in https://bugs.openjdk.org/browse/JDK-8292957

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Commit messages:
 - Remove redundant MemoryLayout::is* methods

Changes: https://git.openjdk.org/panama-foreign/pull/714/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=714&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292957
  Stats: 60 lines in 8 files changed: 1 ins; 42 del; 17 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/714.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/714/head:pull/714

PR: https://git.openjdk.org/panama-foreign/pull/714