Our gdbus generated types use the same names as their corresponding
"real" types, but with "NM" changed to "NMDBus".

Unfortunately, that means that introspection/nmdbus-manager.c (the
generated type for src/nm-manager.c) uses the same type name as the
entirely unrelated src/nm-dbus-manager.c.

Fix this by removing the "d" from src/nm-dbus-manager.c. (We could
rename the generated type instead, but then it becomes inconsistent
with all the other generated types, and we're already using it as
"NMDBusManager" in libnm/nm-manager.c.)