Revert the heinous sysinc hack qdoc needed for clang 3.9

This reverts commits c95cba39 and
65d81734

.  Apparently clang is now
smart enough to handle its system include paths itself; and trying to
"help" was causing problems.  It was, in any case, a heinous hack.

Task-number: QTBUG-66746
Change-Id: I485c6acddfd89f91a4b719f2e2e049ff76a61ce9
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>