simplify `m clean`

`rm -rf out` used to fail because bazel would not set write permission on some files. Now it's been fixed and thus there is little reason to prefer `m clean` (also users are accustomed to `rm -rf out`)

Bug: NA
Test: verified `m clean` works;interestingly soong_metrics file is created at the end
Change-Id: I000d16508613045811fc7792e5798f7c150dcc05
diff --git a/tests/soong_test.sh b/tests/soong_test.sh
index f7bee40..6779d8a 100755
--- a/tests/soong_test.sh
+++ b/tests/soong_test.sh
@@ -9,12 +9,8 @@
 function test_m_clean_works {
   setup
 
-  # Create a directory with files that cannot be removed
-  mkdir -p out/bad_directory_permissions
-  touch out/bad_directory_permissions/unremovable_file
-  # File permissions are fine but directory permissions are bad
-  chmod a+rwx out/bad_directory_permissions/unremovable_file
-  chmod a-rwx out/bad_directory_permissions
+  mkdir -p out/some_directory
+  touch out/some_directory/some_file
 
   run_soong clean
 }