Twenty Fifteen: Remove code and checks for obsolete browsers.

Remove IE specific code from Twenty Fifteen. Remove custom IE stylesheet contents, the HTML5 shiv contents (also targeting Safari 4.x and Firefox 3.x), and IE-specific code. 

Props desrosj, sabernhardt, neychok, joedolson.
See #58836.

git-svn-id: https://develop.svn.wordpress.org/trunk@60964 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson