api: undo go1.txt additions made by 27d0a516b7eb

We don't change go1.txt.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6206080

api/go1.txt

@@ -5484,7 +5484,6 @@ pkg regexp/syntax, const ErrMissingBracket ErrorCode
 pkg regexp/syntax, const ErrMissingParen ErrorCode
 pkg regexp/syntax, const ErrMissingRepeatArgument ErrorCode
 pkg regexp/syntax, const ErrTrailingBackslash ErrorCode
-pkg regexp/syntax, const ErrUnexpectedParen ErrorCode
 pkg regexp/syntax, const FoldCase Flags
 pkg regexp/syntax, const InstAlt InstOp
 pkg regexp/syntax, const InstAltMatch InstOp