cmd/compile: remove incannedimport

cmd/compile: remove incannedimport

This used to be used to give special semantics to the builtin
definitions of package runtime and unsafe, but none of those are
relevant anymore:

- The builtin runtime and unsafe packages do not risk triggering false
  import cycles since they no longer contain `import "runtime"`.

- bimport.go never creates ODCLTYPE, so no need to special case them.

- "incannedimport != 0" is only true when "importpkg != nil" anyway,
  so "incannedimport == 0 && importpkg == nil" is equivalent to just
  "importpkg == nil".

Change-Id: I076f15dd705d4962e7a4c33972e304ef67e7effb
Reviewed-on: https://go-review.googlesource.com/29084
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>