Fix test flake in MdnsDiscoveryManagerTests

The MdnsDiscoveryManagerTests#testDiscoveryExecutor test is flaky
because the Future task sometimes fails to complete on the CF
target. This performance-dependent issue causes Future#isDone to
return a false value. Therefore, remove the isDone check, as
Future#get already performs the same check.

Bug: 392995312
Test: atest MdnsDiscoveryManagerTests#testDiscoveryExecutor \
      --iteration 100
Change-Id: I6a032e80da76eae5ec81bfe7900c0b0e3a3c6efa
1 file changed