chore(config): drop four properties whose value has no effect

[marevol]

Stacked on #3434 — review that one first; this PR's diff against it is 3 files.

#3434 documented these four keys with a "Not read: …" caveat. This removes them instead,
which is what the caveat was really saying.

The four

key	why it does nothing
theme.allowed.archive.extensions	getThemeAllowedArchiveExtensions() has no caller. AdminThemeAction.hasZipExtension hardcodes endsWith(".zip").
theme.assets.cache.max.age	getThemeAssetsCacheMaxAgeAsInteger() has no caller. StaticThemeResponder sends a literal public, max-age=86400 — equal to the default, so setting the key looks harmless and changes nothing.
theme.assets.precompressed	isThemeAssetsPrecompressed() has no caller and there is no precompressed asset path; the responder sets Vary: Accept-Encoding but negotiates nothing.
rag.chat.message.max.length	ChatApiHelper reads it through FessProp#getSystemProperty, which consults conf/system.properties, then -Dfess.system.<key>, then a literal "4000". It never reads fess_config.properties.

The first three are dead outright. The fourth is a live setting declared in the wrong
file — it keeps working exactly as before through conf/system.properties; only the
inert declaration goes.

That last one is not a judgement call. 44 keys are read via getSystemProperty, and
this was the only one also declared in fess_config.properties. The other 43 —
saml.*, spnego.*, oic.*, entraid.*, thumbnail.command.*, rag.llm.name,
api.chat.rate.limit.per.user.per.minute — are not. Removing it makes it consistent
rather than exceptional.

Nothing referenced them

A sweep of every repository in the workspace (*.java, *.jsp, *.xml, *.properties,
*.js, *.ts, *.rst, *.md, *.yaml) found the three theme keys only in
fess_config.properties and the generated FessConfig.java. No plugin, no JSP, no test,
no documentation page.

rag.chat.message.max.length keeps its documentation: fess-docs config/rag-chat.rst and
api/api-chat.rst already describe it in all seven languages, and already say "This value
is read as a System Property; the entry in fess_config.properties is not used." After
this, that sentence can lose its second half — a small fess-docs follow-up, not a gap.

What removal touched

Removing a key also removes what freegen generates from it, so FessConfig still matches
what a regeneration would emit:

	before	after
keys in fess_config.properties	639	635
String X = "…" constants	581	577
defaultMap.put entries	584	580

Exactly −4 in each, plus the 7 accessor declarations and 7 SimpleImpl bodies those keys
generated. keys-without-constant is 58 before and after — a pre-existing property of the
file, untouched here.

Also corrects ChatApiHelper#getMaxMessageLength's javadoc, which said the value came
"from fess_config system properties" and named the channel that does not apply.

Verification

• mvn compile — BUILD SUCCESS.
• mvn test for ChatApiHelperTest, ChatHandlerTest, FessConfigTest and the theme
  suites — 307 tests, 0 failures, 0 errors. The chat tests exercise the surviving
  getSystemProperty path, so the message-length limit is covered as still working.
• mvn formatter:format && mvn license:format — no further changes.
• Grep for each key and each generated constant across src/ — the only survivors are the
  live rag.chat.message.max.length consumer, its tests, and doc comments naming it.