release-24.3: sql: simplify generic query plans

[mgartner]

Backport 4/4 commits from #131791.

/cc @cockroachdb/release

---

sql: do not cache non-reusable plans

Fixes #132963

Release note (bug fix): A bug has been fixed that caused non-reusable
query plans, e.g., plans for DDL and SHOW ... statements, to be cached
and reused in future executions, possibly causing stale results to be
returned. This bug only occurred when plan_cache_mode was set to
auto or force_generic_plan, both of which are not currently the
default settings.

sql: remove legacy planning code path

An optimization code path from before the introduction of generic query
plans has been removed. This code path remained in the codebase to
de-risk the generic query plans backports. It is not needed in future
versions.

Release note: None

sql: remove unused context parameter in reuseMemo

Release note: None

sql: simplify generic query plans

All fully-optimized memos that can be reused without re-optimization are
now stored in PreparedStatement.GenericMemo. Prior to this commit,
some fully-optimized memos were stored in PreparedStatement.BaseMemo.

Release note: None

---

Release justification: Refactoring that narrowly missed branch-cut.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[DrewKimball]

Reviewed 2 of 2 files at r1, 1 of 1 files at r2, 1 of 1 files at r3, 2 of 2 files at r4, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @mgartner)