fix error introduced in !230

Just looked at the code again and noticed that there is no stream in this instruction set anymore, so this line would have raised an error.

pystencils/backends/ppc_instruction_sets.py

@@ -78,7 +78,6 @@ def get_vector_instruction_set_ppc(data_type='double', instruction_set='vsx'):
         # Clang and XL C++ are missing these for doubles
         result['loadA'] = '(__vector double)' + result['loadA'].format('(float*) {0}')
         result['storeA'] = result['storeA'].format('(float*) {0}', '(__vector float) {1}')
-        result['stream'] = result['stream'].format('(float*) {0}', '(__vector float) {1}')
         result['storeAAndFlushCacheline'] = result['storeAAndFlushCacheline'].format('(float*) {0}',
                                                                                      '(__vector float) {1}')