browsertests: Enable VE on IE and Safari; remove Chrome on Windows XP

VisualEditor is about to be made available on IE11 (with IE10 to come),
so it makes sense to start browser testing it beforehand. It's been
running in Safari with some rare Safari-only bugs that we want to detect
more usefully. However, the Windows XP build of Chrome hasn't flagged
anything unique in all its runs, and so adds no value.

Deleted job:

browsertests-VisualEditor-en.wikipedia.beta.wmflabs.org-windows_xp-firef
ox-sauce

Created jobs:

browsertests-VisualEditor-en.wikipedia.beta.wmflabs.org-os_x_10.9-safari
-sauce
browsertests-VisualEditor-en.wikipedia.beta.wmflabs.org-windows_8-intern
et_explorer-sauce
browsertests-VisualEditor-test2.wikipedia.org-os_x_10.9-safari-sauce
browsertests-VisualEditor-test2.wikipedia.org-windows_8-internet_explore
r-sauce

Bug: 70126
Change-Id: I5c5f42c38f35880a8eeffd4696650cf83aec04f1