Revert "Making CompletableFutureTest as known failure on gcstress debug"

This reverts changes I5845b79dd85dae3ce3ab5914761fc54b8f1da4f0
and I4f06af6ad38ea6ecb39cbb160ef5862cf3d12f87.

The fix in I0846ed1aebe93e4a3fbb9c7189f73bd7f86357e6 that adds a
delay factor for gcstress tests is enough to allow enough time
for the test to complete.

Fixes: 205007075
Test: art/tools/run-libcore-tests.sh --mode host --gcstress \
      --variant X32 --debug jsr166.CompletableFutureTest
      # ran with and without the delay factor for gcstress
Change-Id: I68d342d26004d85f7ae546c9eeb1034f6f9e430b