std: remove unused bits of code all over the place

Some were never used, and some haven't been used for years.

One exception is net/http's readerAndCloser, which was only used in a
test. Move it to a test file.

While at it, remove a check in regexp that could never fire; the field
is an uint32, so it can never be negative.

Change-Id: Ia2200f6afa106bae4034045ea8233b452f38747b
Reviewed-on: https://go-review.googlesource.com/c/go/+/192621
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

decode_test.go

@@ -1744,41 +1744,6 @@ type NullTest struct {
 	Struct struct{}
 }
 
-type NullTestStrings struct {
-	Bool      bool              `json:",string"`
-	Int       int               `json:",string"`
-	Int8      int8              `json:",string"`
-	Int16     int16             `json:",string"`
-	Int32     int32             `json:",string"`
-	Int64     int64             `json:",string"`
-	Uint      uint              `json:",string"`
-	Uint8     uint8             `json:",string"`
-	Uint16    uint16            `json:",string"`
-	Uint32    uint32            `json:",string"`
-	Uint64    uint64            `json:",string"`
-	Float32   float32           `json:",string"`
-	Float64   float64           `json:",string"`
-	String    string            `json:",string"`
-	PBool     *bool             `json:",string"`
-	Map       map[string]string `json:",string"`
-	Slice     []string          `json:",string"`
-	Interface interface{}       `json:",string"`
-
-	PRaw    *RawMessage           `json:",string"`
-	PTime   *time.Time            `json:",string"`
-	PBigInt *big.Int              `json:",string"`
-	PText   *MustNotUnmarshalText `json:",string"`
-	PBuffer *bytes.Buffer         `json:",string"`
-	PStruct *struct{}             `json:",string"`
-
-	Raw    RawMessage           `json:",string"`
-	Time   time.Time            `json:",string"`
-	BigInt big.Int              `json:",string"`
-	Text   MustNotUnmarshalText `json:",string"`
-	Buffer bytes.Buffer         `json:",string"`
-	Struct struct{}             `json:",string"`
-}
-
 // JSON null values should be ignored for primitives and string values instead of resulting in an error.
 // Issue 2540
 func TestUnmarshalNulls(t *testing.T) {