Remove ubuntu1604 from presubmit.yml (#105)

Ubuntu 16.04 is end-of-life, we're going to remove it from Bazel CI.

If you like you can add testing on `ubuntu2004` platform which we also support.
diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index a4dd49c..e469387 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -1,24 +1,19 @@
 ---
 platforms:
-  ubuntu1604:
-    build_targets:
-    - "..."
-    test_targets:
-    - "..."
   ubuntu1804:
     build_targets:
-    - "..."
+    - "//..."
     test_targets:
-    - "..."
+    - "//..."
   macos:
     build_targets:
-    - "..."
+    - "//..."
     test_targets:
-    - "..."
+    - "//..."
   windows:
     build_targets:
-    - "..."
+    - "//..."
     test_targets:
-    - "..."
+    - "//..."
 
 buildifier: latest