Remove (or implement) unused config section constants and non-functional documented config keys

[xrpl365]

Summary

A code-first audit of the configuration system found a few config items that are
either defined in code but never read, or documented in cfg/xrpld-example.cfg
but never parsed. None of them have any runtime effect today. This issue asks for
a decision on each: remove as dead, or implement the intended behavior.

These were deliberately left out of the accompanying documentation PRs because
removing code is a maintainer judgement call, not a documentation fix.

Findings (all verified against develop)

1. kSntp ("sntp_servers"): unused constant

• Defined at include/xrpl/config/Constants.h:59 as
  static constexpr auto kSntp = "sntp_servers";
• No reader anywhere in src/ (only the definition exists). There is no
  SNTP/NTP client in the codebase; TimeKeeper only references SNTP in a comment.
• A sample config in src/test/core/Config_test.cpp:94 contains a literal
  [sntp_servers] block, but it is never asserted against and does not use the
  constant.

Question: Was [sntp_servers] an intended (never-finished) feature, or is it
safe to remove the constant?

2. kRelationalDb ("relational_db"): unused constant, superseded by [sqdb]

• Defined at include/xrpl/config/Constants.h:52 as
  static constexpr auto kRelationalDb = "relational_db";
• No reader anywhere in src/. The relational backend selector is actually read
  from the [sqdb] section at src/libxrpl/rdb/SociDB.cpp:55
  (config.section(Sections::kSqdb)).
• The related README inaccuracy (rdb/README.md referencing [relational_db]) is
  being corrected separately as a documentation fix, independent of this
  issue, since the code reads [sqdb] today regardless of this decision.

Question: Remove the unused relational_db constant, or is a future
[relational_db] backend selector planned?

3. [websocket_ping_frequency]: documented, never read

• Documented at cfg/xrpld-example.cfg:360 (a [websocket_ping_frequency]
  section with a description of WebSocket ping interval).
• No code reads this section. Setting it has no effect.

Question: Implement the WebSocket ping interval, or remove the documentation?

4. transaction_queue.zero_basefee_transaction_feelevel: documented, never read

• Documented at cfg/xrpld-example.cfg:681 (under [transaction_queue], default
  256000).
• Not present in TxQ::Setup and not parsed in setup_TxQ. Setting it has no
  effect.

Question: Implement the zero-base-fee fee-level override, or remove the
documentation?

Suggested action

If confirmed dead, a single small PR can:

• Remove kSntp and kRelationalDb from include/xrpl/config/Constants.h.
• Remove the [websocket_ping_frequency] and zero_basefee_transaction_feelevel
  blocks from cfg/xrpld-example.cfg.

If any are intended features, this issue can be repurposed to track their
implementation instead.

Notes

• Verified no source references the two constants (only their definitions exist).
• This issue intentionally excludes documentation corrections and the addition of
  undocumented-but-functional settings, which are handled as separate
  documentation PRs.