[dev.regabi] cmd/compile: remove Sym.Importdef

Evidently it hasn't been needed since circa 2018, when we removed the
binary export data format.

Change-Id: I4e4c788d6b6233340fb0de0a56d035c31d96f761
Reviewed-on: https://go-review.googlesource.com/c/go/+/280634
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/typecheck/export.go

@@ -59,7 +59,6 @@ func importsym(ipkg *types.Pkg, pos src.XPos, s *types.Sym, op ir.Op, ctxt ir.Cl
 	n := ir.NewDeclNameAt(pos, op, s)
 	n.Class_ = ctxt // TODO(mdempsky): Move this into NewDeclNameAt too?
 	s.SetPkgDef(n)
-	s.Importdef = ipkg
 	return n
 }
 

src/cmd/compile/internal/types/sizeof_test.go

@@ -20,7 +20,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Sym{}, 48, 80},
+		{Sym{}, 44, 72},
 		{Type{}, 56, 96},
 		{Map{}, 20, 40},
 		{Forward{}, 20, 32},

src/cmd/compile/internal/types/sym.go

@@ -27,7 +27,6 @@ import (
 // NOTE: In practice, things can be messier than the description above
 // for various reasons (historical, convenience).
 type Sym struct {
-	Importdef *Pkg   // where imported definition was found
 	Linkname string // link name
 
 	Pkg  *Pkg