strconv: remove needless cast

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/119340043

src/pkg/strconv/atoi.go

@@ -31,7 +31,7 @@ func rangeError(fn, str string) *NumError {
 	return &NumError{fn, str, ErrRange}
 }
 
-const intSize = 32 << uint(^uint(0)>>63)
+const intSize = 32 << (^uint(0) >> 63)
 
 // IntSize is the size in bits of an int or uint value.
 const IntSize = intSize