Remove -ms-linear-gradient from .vertical-gradient LESS mixin

Comment claims that it's for IE 10, but it's not the case. IE 10 supports
it without a vendor prefix, and IE < 10 doesn't support it at all
(tested on Windows Phone 8). Possibly a non-final version of IE 10 needed
it.

https://developer.mozilla.org/en-US/docs/Web/CSS/linear-gradient
http://caniuse.com/css-gradients

Change-Id: I09add76ef5db3a990a8769ea8771bfa7931ebc29
Merge "Remove -ms-linear-gradient from .vertical-gradient LESS mixin"