[foreign-memaccess+abi] RFR: 8292957: Remove redundant MemoryLayout::is* methods [v2]

[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

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Revoke import optimizations

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/714/files
  - new: https://git.openjdk.org/panama-foreign/pull/714/files/19371077..49dc8ee4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=714&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=714&range=00-01

  Stats: 25 lines in 3 files changed: 20 ins; 1 del; 4 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