Remove workaround for https://github.com/fsharp/fsharp/issues/190 in numerics tests

The Mono bug this is working around was fixed in late 2013, and is in the Mono 3.2 series
that F# already strongly recommends.

The workaround causes the unit-tests to fail on a Mono runtime which does not exhibit
https://github.com/fsharp/fsharp/issues/190, so this fixes the tests.