Effectively EOL testing.NullLogger

[thockin]

It remains as a type, but is just an alias to DiscardLogger.

Fixes #33

I could be talked into simply deleting the type - thoughts?

@DirectXMan12

[wojas]

I would slightly favor keeping it in case someone uses it, but I am OK either way.

godoc suggests some special syntax for deprecation:

Sometimes a struct field, function, type, or even a whole package becomes redundant or unnecessary, but must be kept for compatibility with existing programs. To signal that an identifier should not be used, add a paragraph to its doc comment that begins with "Deprecated:" followed by some information about the deprecation. There are a few examples in the standard library.

https://blog.golang.org/godoc

Example:

// Deprecated: use logr.DiscardLogger instead

This does not result in any special markup on godoc, but may be supported by certain linters and IDEs. The InfoLogger deprecation notice also uses this.

[DirectXMan12]

Deprecated is fine with me. We actually ended up copying NullLogger into CR because it was in a test package. I should switch us to use DiscardLogger instead :-).