cmd/link: remove dead importcycles function

The importcycles method has not been useful since April 2016 when a
large code deletion was performed.

The compiler itself provides some protection against import cycles, and
the linker now does import cycle detection in linksetup -> postorder.

For #57400

src/cmd/link/internal/ld/go.go

@@ -452,48 +452,3 @@ func (ctxt *Link) addexport() {
 		adddynlib(ctxt, lib)
 	}
 }
-
-type Pkg struct {
-	mark    bool
-	checked bool
-	path    string
-	impby   []*Pkg
-}
-
-var pkgall []*Pkg
-
-func (p *Pkg) cycle() *Pkg {
-	if p.checked {
-		return nil
-	}
-
-	if p.mark {
-		nerrors++
-		fmt.Printf("import cycle:\n")
-		fmt.Printf("\t%s\n", p.path)
-		return p
-	}
-
-	p.mark = true
-	for _, q := range p.impby {
-		if bad := q.cycle(); bad != nil {
-			p.mark = false
-			p.checked = true
-			fmt.Printf("\timports %s\n", p.path)
-			if bad == p {
-				return nil
-			}
-			return bad
-		}
-	}
-
-	p.checked = true
-	p.mark = false
-	return nil
-}
-
-func importcycles() {
-	for _, p := range pkgall {
-		p.cycle()
-	}
-}

src/cmd/link/internal/ld/lib.go

@@ -646,8 +646,6 @@ func (ctxt *Link) loadlib() {
 	// We've loaded all the code now.
 	ctxt.Loaded = true
 
-	importcycles()
-
 	strictDupMsgCount = ctxt.loader.NStrictDupMsgs()
 }