Delete net_test from system/extras now that it's in kernel/tests.

The directories are identical. This commit was generated by:

$ diff -u system/extras/tests/net_test kernel/tests/net/test/
$ cd system/extras/tests/net_test
$ git rm *

Bug: 27228782
Change-Id: I08ca870049c8e91bb0848314bf647f6e1ad2dd09
23 files changed