WrappedStringList: Remove unused protected `extend()` method
WrappedStringList: Remove unused protected `extend()` method

This was accidentally left behind in 36459482b0 (Idbfb609aa95).
Found via doc.wikimedia.org code coverage report, showing this method
as uncovered.

Change-Id: I490cd8780824bf14dfa0f5271fd8041e1db2008b

wikimedia/WrappedString