Twenty Fourteen: Remove code and checks for obsolete browsers.

Remove IE specific code from Twenty Fourteen. Remove custom IE stylesheet contents, the HTML5 shiv contents (also targeting Safari 4.x and Firefox 3.x), and IE-specific code. Change relevant enqueues to registrations.

Props desrosj, sabernhardt, karmatosed, neychok, metodiew, mukesh27, joedolson.
See #58836.

git-svn-id: https://develop.svn.wordpress.org/trunk@60963 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson