cmd/compile: remove types.Sym.lsym

It was just a cache, and the CL series yesterday
removed 40% of the calls to types.Linksym in make.bash.

Testing atop CL 40693 (backend concurrency)
indicates that removing it is actually a very minor
performance improvement.

Passes toolstash-check.

Change-Id: I97c2973036964acdd11b3cb842bc31f33ae60389
Reviewed-on: https://go-review.googlesource.com/41492
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -22,7 +22,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Sym{}, 56, 96},
+		{Sym{}, 52, 88},
 		{Type{}, 52, 88},
 		{Map{}, 20, 40},
 		{Forward{}, 20, 32},

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

@@ -31,7 +31,6 @@ type Sym struct {
 	flags   bitset8
 	Label   *Node // corresponding label (ephemeral)
 	Origpkg *Pkg  // original package for . import
-	lsym    *obj.LSym
 }
 
 const (
@@ -78,8 +77,5 @@ func (sym *Sym) Linksym() *obj.LSym {
 	if sym == nil {
 		return nil
 	}
-	if sym.lsym == nil {
-		sym.lsym = Ctxt.Lookup(sym.LinksymName())
-	}
-	return sym.lsym
+	return Ctxt.Lookup(sym.LinksymName())
 }