Remove legacy index detection from BulkWrite.c and our use of the undocumented "legacyIndex" index option in libmongoc (removed by CDRIVER-2243).

The following tests refer to MongoDB 2.4 and can be removed:

• tests/bulk/bulkwrite-insert-002.phpt
• tests/bulk/bulkwrite-insert-003.phpt (not directly 2.4 related but it exercises the legacy index code path on 2.6+)
• tests/writeResult/writeresult-getmodifiedcount-003.phpt

STANDALONE_24 can also be removed from tests/utils/basic.inc and the Vagrant configuration (we no longer need to launch 2.4 servers).

There may be other areas of code for us to clean up. Looking through the related commits in CDRIVER-2243 should indicate those bits.