Partial fix #2765: Remove long deprecated declaration in ScalaNativePlugin

[LeeTibbert]

PR #2783 removed a long deprecated declaration in ScalaNativePlugin.scala. For reasons
at the time, it was never merged. However changelog/0.5.0.md was changed to note
the deprecation.

This PR removes that deprecated declaration. There is no need to update the documentation.

[LeeTibbert]

When I updated changelog/0.5.0.md weeks ago, I did not create a "suggested replacement" line
because the original deprecation message made no sense at all to me and I did not have
the patience to go diving into long obsolete sbt practice.

@deprecated("use autoImport instead", "0.3.7")
  val AutoImport = autoImport

Note the case differences.

If anyone can suggest text for a sensible "Suggested replacement" section, I can update the changelog.
By now, anybody using the deprecated declaration (or the "use..." text") is probably long retired.

[WojciechMazur]

I don't think any suggested replecement is really needed. Usage of autoImport object is a standard behavior in most of the sbt plugins.