Remove obsolete PV_PYMODULE_BUILD_DIR global property.

PV_PYMODULE_BUILD_DIR is not used by Python modules in VTK/ParaView. Remove
that. Instead fixing the logic to consistently rely on
VTK_BUILD_PYTHON_MODULE_DIR and VTK_INSTALL_PYTHON_MODULE_DIR.

Change-Id: Ieac186e19058bba8738d0f577d2a81dbc3979065
Merge topic 'fix_python_module_rules' into next

74059a2 Remove obsolete PV_PYMODULE_BUILD_DIR global property.