Clean up init section

[808codist]

This whole init section could be cleaned up:

1. As you noted _validTagValueRegexp is not used. So we could remove it. @wblakecaldwell does this regex have a purpose?
2. Since the regex is a static string, we can also use regexp.MustCompile() which does not return an error and will panic if the regex does not compile. But this is exactly what initTagBatch() is doing so we can simplify the declaration to var validTagValueRegexp = regexp.MustCompile(`^[a-zA-Z0-9_-]+$`) and remove initTagBatch() completely.
3. Each Go file may have its own init() function [1]. I don't think we need init.go to call a bunch of other init functions. Unless there's some dependency and the order is important. But in this case I don't think it does.

[1] https://go.dev/doc/effective_go#init

Originally posted by @cleroux in #17 (comment)

[wblakecaldwell]

Ah, I didn't know you could have an init() function in each file.

Hah - yeah... ideally, we could do more validation here, and use that regex, but removing it is fine.

Good call