capn-test: remove unreferenced code, now builds cleanly
capn-test: remove unreferenced code, now builds cleanly

previously, this was emitting the following warning (treated as error):

`c++ -g -Wall -Werror -I. -Igtest-1.7.0/include -o capn-test.o -c capn-test.cpp
capn-test.cpp:66:29: error: unused variable 'SUBSTRUCT_DEFAULT' [-Werror,-Wunused-const-variable]
static const AlignedData<2> SUBSTRUCT_DEFAULT = {{0,0,0,0,1,0,0,0,  0,0,0,0,0,0,0,0}};
                            ^
capn-test.cpp:67:29: error: unused variable 'STRUCTLIST_ELEMENT_SUBSTRUCT_DEFAULT' [-Werror,-Wunused-const-variable]
static const AlignedData<2> STRUCTLIST_ELEMENT_SUBSTRUCT_DEFAULT =
                            ^`
liamstask/c-capnproto,fluffysquirrels/c-capnproto,liamstask/c-capnproto,opensourcerouting/c-capnproto,fluffysquirrels/c-capnproto,liamstask/c-capnproto,liamstask/c-capnproto,opensourcerouting/c-capnproto