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