Removing includes which were not needed by headers
Note that if you include those headers directly (instead of including irrlicht.h) you may have to add some new includes now.
Thought I generally tried to avoid removing headers where it leads to too much changes in user-code
Reason was mainly that IntelliSense in VisualStudio 17.7 added a new feature which shows those unused headers :)
Has to be used a bit careful as it doesn't know about defines and other platforms
And I only did clean up public headers so far (will probably do some more later or another time)
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6524 dfc29bdd-3216-0410-991c-e03cc46cb475
