Merge #21415: refactor: remove Optional & nullopt

ebc4ab721b refactor: post Optional<> removal cleanups (fanquake)
57e980d13c scripted-diff: remove Optional & nullopt (fanquake)

Pull request description:

  Same rationale & motivation as #21404, which turned out to be quite low in the number of potential conflicts. Lets see what the bot has to say here.

ACKs for top commit:
  practicalswift:
    cr ACK ebc4ab721b: patch looks correct
  jnewbery:
    utACK ebc4ab721b
  laanwj:
    Code review ACK ebc4ab721b

Tree-SHA512: 550fbeef09b9d35ddefaa805d1755c18c8fd499c4b0f77ebfece8c20296a7abd1cf6c699e2261f92fe3552deeb7555ec2a2287ffe3ab9e98bb9f8612a4d43be3

src/Makefile.am

@@ -182,7 +182,6 @@ BITCOIN_CORE_H = \
   node/ui_interface.h \
   node/utxo_snapshot.h \
   noui.h \
-  optional.h \
   outputtype.h \
   policy/feerate.h \
   policy/fees.h \