Remove instructions about running post-submit tests with Atest.
These have become obsolete, as ART's `TEST_MAPPING` file no
longer contains a `postsubmit` test group.
Test: Render `README.atest.md` as HTML and check the output manually
Change-Id: I903249a02b277f56c7ef8507d55fb747f364c652
diff --git a/test/README.atest.md b/test/README.atest.md
index ea6685b..c23a7d6 100644
--- a/test/README.atest.md
+++ b/test/README.atest.md
@@ -170,8 +170,3 @@
```bash
atest --test-mapping art:presubmit
```
-
-To only run tests executed during post-submit testing, use:
-```bash
-atest --test-mapping art:postsubmit
-```