Remove linker options for processing warnings as errors
This fixes errors encountered when compiling VTK with some gcc 4.9.2 compilers that generates false positive.
A new advanced option VTK_LINKER_FATAL_WARNINGS to turn ON the flag as it used to be.
More details on the issue is provided in the following thread of the VTK-dev mailing list: http://vtk.1045678.n5.nabble.com/Issue-with-Linux-extra-linker-flags-td5732819.html