Don't prune $(OUT_DIR) from findleaves.py

Pruning $(OUT_DIR) can be overaggressive if $(OUT_DIR) matches the name
of any directory in the source tree that contains Android.mk files.
Pruning $(OUT_DIR) is no longer necessary, as one of the first things
the build system does is put an empty Android.mk file in there to avoid
recursing into it.

Bug: 31941856
Test: m -j OUT_DIR=angler
Change-Id: I7c5e95816f35bd5906845716d56cca76de7fa1ad