Removed stale references to Mongoid.unit_of_work in benchmark suite, which was removed with Identity Map

Currently, `perf/benchmark.sh` fails with the following errors:

````
Creating indexes...
Starting benchmark...
       user     system      total        real

[ Root Document Benchmarks ]
perf/benchmark.rb:21:in `block (2 levels) in <main>': undefined method `unit_of_work' for Mongoid:Module (NoMethodError)
	from perf/benchmark.rb:19:in `each'
	from perf/benchmark.rb:19:in `block in <main>'
	from /Users/marms/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/benchmark.rb:174:in `benchmark'
	from /Users/marms/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/benchmark.rb:206:in `bm'
	from perf/benchmark.rb:15:in `<main>'
````

This must have broken in the last commit 718017394fe3b04b75f13e91e2dabd524106c577, "Remove IdentityMap for good".

To fix, I removed `Mongoid.unit_of_work` blocks, and de-indented accordingly.