Cleanup unused accessor methods inf REXML::Functions

[tompng]

Removes unused Functions.variables. It was write only, never read.
Removes Functions.namespace_context. It was only used from QuickPath, and QuickPath has been removed in #362

[Copilot]

Pull request overview

This PR cleans up dead state/accessors in REXML::FunctionsClass and removes now-unnecessary plumbing from the XPath parsers, following the earlier removal of QuickPath (#362).

Changes:

• Removes the unused FunctionsClass#variables and #namespace_context accessors and their internal state.
• Stops propagating namespaces/variables into FunctionsClass from both REXML::XPathParser and REXML::Parsers::XPathParser.
• Tightens the singleton thread-safety note to reflect the remaining shared state.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/rexml/xpath_parser.rb	Stops writing namespaces/variables into @functions; keeps variables coercion local to the parser.
lib/rexml/parsers/xpathparser.rb	Removes the (now-unused) namespace propagation into Functions.
lib/rexml/functions.rb	Removes unused accessor methods/state and updates the singleton thread-safety comment.

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