cmd/compile: delete bitset16

It is no longer used.

Change-Id: Id64f387867a0503d13eaecda12e6606682c24595
Reviewed-on: https://go-review.googlesource.com/41790
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/bitset.go

@@ -14,16 +14,6 @@ func (f *bitset8) set(mask uint8, b bool) {
 	}
 }
 
-type bitset16 uint16
-
-func (f *bitset16) set(mask uint16, b bool) {
-	if b {
-		*(*uint16)(f) |= mask
-	} else {
-		*(*uint16)(f) &^= mask
-	}
-}
-
 type bitset32 uint32
 
 func (f *bitset32) set(mask uint32, b bool) {