Remove broken URL from libstdc++ manual

The URL for the "What Are Allocators Good For?" article has been a
recurring source of problems. It moved from the C/C++ Users Journal
website to the Dr Dobbs site after CUJ shut down, and the original
domain changed hands, leaving old links pointing to nefarious sites.

Now the URL to the copy on drdobbs.com no longer works either and I
can't find a (legal) copy of the article online. The simplest solution
is to remove the URL.

	* doc/xml/manual/allocator.xml: Remove URL for bibliography entry.
	* doc/html/*: Regenerate.

From-SVN: r275398