Revert "go/ast: clarify when Ident.Obj is nil"
Revert "go/ast: clarify when Ident.Obj is nil"

This reverts commit 52aef05498a9e84ede16fb7ce46a2a252af05479.

Reason for revert: After discussion on CL 347530, it is not clear this is an improvement to the documentation.

Updates #48141

Change-Id: I5f3d9995c5f5666b92602c4b8ec393673baa73fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/347592
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go