writeback_bulk_insert.js builds object that is too large

We did this purposely to check an edge case in writebacks, but now it's impossible to use the shell this way after https://github.com/mongodb/mongo/commit/98f35d3ff6e4b6b547618eae26936e69ebff45c0.

Since the WBL is being deprecated, suggest removing this test case, as it's no longer trigger-able from the shell.