Remove gb2312 encoding as it can crash the program

Upstream go's x/text/encoding package has a bug that makes the
'hz-gb-2312' encoding crash (golang/go#35118)

This was reported to go-message in
emersion#95

This commit removes the offending encoding so that go-message can't crash if
people import the charset package. It should be reverted once the upstream
package is fixed by the go developers.

charset/charset.go

@@ -13,7 +13,6 @@ import (
 	"golang.org/x/text/encoding"
 	"golang.org/x/text/encoding/charmap"
 	"golang.org/x/text/encoding/ianaindex"
-	"golang.org/x/text/encoding/simplifiedchinese"
 )
 
 // Quirks table for charsets not handled by ianaindex
@@ -33,8 +32,7 @@ var charsets = map[string]encoding.Encoding{
 	"cp367":            encoding.Nop,
 	"ascii":            encoding.Nop, // non-standard
 
-	"ansi_x3.110-1983": charmap.ISO8859_1,     // see RFC 1345 page 62, mostly superset of ISO 8859-1
-	"gb2312":           simplifiedchinese.GBK, // GBK is a superset of HZGB2312
+	"ansi_x3.110-1983": charmap.ISO8859_1, // see RFC 1345 page 62, mostly superset of ISO 8859-1
 	"cp1250":           charmap.Windows1250,
 	"cp1251":           charmap.Windows1251,
 	"cp1252":           charmap.Windows1252,

charset/charset_test.go

@@ -45,11 +45,6 @@ var testCharsets = []struct {
 		charset: "idontexist",
 		encoded: []byte{42},
 	},
-	{
-		charset: "gb2312",
-		encoded: []byte{178, 226, 202, 212},
-		decoded: "测试",
-	},
 }
 
 func TestCharsetReader(t *testing.T) {