Rewrite the remove_during_mr.js test as an FSM workload

Kamran used jstests/libs/parallelTester.js explicitly in the remove_during_mr.js test in order to create a stand-alone script to trigger the invariant failure described in SERVER-15539. It is straightforward to model having "map-reduce clients" and "remove clients" executing concurrently as a FSM workload. Rewriting the remove_during_mr.js in this fashion also makes it simple to tune the test based on the existing map_reduce*.js FSM workloads and avoid having it take a very long time on certain build variants (in particular, the OS X and ARM builders).