Replace update_web_docs_libstdcxx_svn with update_web_docs_libstdcxx_git

This patch replaces the update_web_docs_libstdcxx_svn script, that
updates online documentation from its sources in the GCC repository, run
once a day from cron, with update_web_docs_libstdcxx_git.

	* update_web_docs_libstdcxx_git: New file.
	* update_web_docs_libstdcxx_svn: Remove.
	* crontab: Use update_web_docs_libstdcxx_git.