Removed -fobjc-gc from VTK_REQUIRED_OBJCXX_FLAGS

There are 3 memory management models in Cocoa,
chronologically by introduction: MRR, GC, ARC.
VTK supports the first two.
GC is now deprecated however, and Xcode 5.1 will
no longer even build GC code.  Therefore
removed the -fobjc-gc option as a default.
(It built the libraries as both MRR and GC
compatible, which was convienent for library code.)

Change-Id: I5f0ee437402bbcbbb20f2088f56298adf3a34d6e