The  index_builds_ignore_prepare_conflicts.js test was disabled by SERVER-43301.

To make this test pass, disable the fix put in by SERVER-38588 to block prepared transactions on index builds on secondaries.

When two-phase index builds are in use, it is both unnecessary and unsafe to block a prepared transactions on index builds on secondaries. Due to locking, we can guarantee that a transaction prepared on a primary during an index build will always commit before that index build completes. Because two-phase index builds replicate start and commit oplog entries, it will never be possible to replicate a prepared transaction, commit an index build, then commit the transaction, the bug SERVER-38588 addressed.

This blocking behavior can also introduce a deadlock with two-phase index builds on a secondary if a prepared transaction blocks on an index build, but the index build can't re-acquire its S or X lock because of the transaction.