clatd.c: remove unsetenv("ANDROID_DNS_MODE")

As of bionic commit I835a7b42d6afbc9c67866594c7951cfd9b355d81,
ANDROID_DNS_MODE is deleted from the environment when a security
transition occurs. As netd and clatd run in different SELinux security
domains, this variable will be automatically deleted. It's unnecessary
(but harmless) to delete it ourselves. Delete unnecessary code.

Test: Android compiles and runs with no obvious problems.
Change-Id: Id46c1a6989a3441a11b4be3d8a28aa1b926c1c6d