Remove erroneous SparseVector early exit

[mbauman]

There is an optimization available here, but this isn't quite it. Ref JuliaLang/julia#14442.

[tkelman]

Thanks. Do you need/want an invite to this org?

[mbauman]

I'm not currently an org member, but I won't be doing any significant work here in the near future, so it's not a big deal. This was simple enough that I could do it in a few minutes.

That said, it'd probably be good to do a little maintenance on this package if folks are using it (I'm not). If it hasn't happened already, this should be deprecated/capped at 0.4. It'd also be great to port my additional commits from JuliaLang/julia#13440 so this matches what ended up in 0.5.

[tkelman]

As I said over in the julia pr, capping it to 0.4 would cause issues for packages that want to use this on 0.4 and the base version on 0.5, unless we branch this package into an 0.4 version and an empty 0.5 stub. That might be the right approach for whoever wants to put in the work.