test-bus-gvariant: remove unused variable
execute.h: remove redefinition of Unit

Unit is typedef'ed in both unit.h and execute.h. The typedef
existed first in unit.h and was later added to execute.h in
c17ec25e4d9bd6c8e8617416f813e25b2ebbafc5

It is no longer needed so let's just keep the one in unit.h to
avoid redefining it.