Delete mssql.go

To support machines with low on ram. Eg. a 512MB DO development box.
Currently, building fails with:
`go build [github.com/denisenkom/go-mssqldb]: signal killed`

We don't use MSSQL, so we don't need support for it.

More on this issue here: https://code.google.com/p/go/issues/detail?id=6251