internal/abi: get rid of itab blank field

We don't need it anymore, as we're accessing all the field offsets
symbolically.
This won't affect 64-bit, but it makes itabs on 32-bit 4 bytes smaller.

Change-Id: I7fb937813111476fd5ab03005b248ea7bb962cf6
Reviewed-on: https://go-review.googlesource.com/c/go/+/549517
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: randall77

src/internal/abi/iface.go

@@ -12,7 +12,6 @@ package abi
 type ITab struct {
 	Inter *InterfaceType
 	Type  *Type
-	Hash  uint32 // copy of Type.Hash. Used for type switches.
-	_     [4]byte
+	Hash  uint32     // copy of Type.Hash. Used for type switches.
 	Fun   [1]uintptr // variable sized. fun[0]==0 means Type does not implement Inter.
 }