Call rm -rf in mixed build build statement preamble.

https://android-review.git.corp.google.com/q/topic:bazel_using_txt added
a tree artifact output for an action. This CL adds "-r" to the rm calls
to support deleting the old tree artifact outputs.

Test: presubmits
Fixes: 257956807
Change-Id: Ia44b1bef4f8031fe75a239fb162972e8827e3c19