Delete runtests.sh, superseded by atest

To run all tests, simply enter system/netd and run atest with no
arguments. Tests to run are listed in TEST_MAPPING, and atest will
rebuild them automatically before installing them on the target. Atest
also works with acloud (aka cuttlefish).

Test: cd system/netd && atest
Change-Id: I3027f405f183e3f0ecb91f1ad831f1f3f3c3ee4c