Remove binder threadpool init from puller tests

LibStatsPullTests used to manually create a binder threadpool. However,
it seems that it is automatically done for us now, so we should not
override the defaults since this can result in shrinking the threadpool:
b/150904425.

Test: atest LibStatsPullTests
Bug: 150904425
Change-Id: I3f1a95aba3681c43fd46a9448c93a71fcb3133a9