Remove obsolete build configuration for QtWebKit

Building the qtwebkit module works the same way as any other module
these days (i.e. qmake && make), thus using the build-webkit script
for this purpose is obsolete.
In particular when using the qt5_tool the module is actually built
twice: first time with the top-level Makefile, and for the second
time in a final step with build-webkit which fails because the other
build is present. Since the build-webkit script is meant for upstream
developer builds the option can be removed from qt5_tool.
Configure with '-skip qtwebkit' when a webkit build was not requested.

Task-number: QTBUG-30858
Change-Id: Ib6b51b188c9f97064efb834da46de4d03e2a72b7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Fix bug when skipping webkit on windows

Do not add "-skip qtwebkit" as one single item to the array, since that
will effectively run configure like this:
.\configure.exe "-skip qtwebkit"

This gave the error:
Unknown option -skip qtwebkit

This was found to be a problem with Windows Powershell, not sure if this
is a problem under command.com

Change-Id: Id2a2e8d4705fc46905450e6a7ac503d6c27710aa
Reviewed-by: Andras Becsi <andras.becsi@digia.com>