Removed repetition of URIs in the title attributes (tooltips) of external links....

Removed repetition of URIs in the title attributes (tooltips) of external links. Web browsers typically display the target URIs of links in the status bar, making repeating the URIs in tooltips redundant. Some browsers, like Opera, can be configured to display the URIs in tooltips instead of the status bar without the web page having to supply title attributes at all.

As part of this process, I refactored some of Linker.php and removed an unused parameter from getExternalLinkAttributes.