Remove IntelliJ IDEA specific files from the source code repository

A DESCRIPTION OF THE PROBLEM :
Remove IDE specific files from the repo because IntelliJ IDEA generates the IDE files from the gradle project definition.

Related: https://bugs.openjdk.java.net/browse/JDK-8198795
Related: https://github.com/javafxports/openjdk-jfx/issues/4

Comment from Kevin Rushforth:
"
I just approved PR #401 which can unblock this. As a next step, JDK-8198795 should be split into 3 separate issues, one for each of IntelliJ, Eclipse, and Netbeans. You will need general consensus from the users of an IDE before removing the checked in files for that IDE. By splitting it into three separate issues, you can take care of each IDE separately when the users of that IDE are happy with it.
"