cmd/doc: don't bother checking validity of symbols at top level

No need to check as pieces further down do so anyway:

	%  go doc '&&.%$^'
	doc: symbol && is not a type in package fmt installed in "fmt"
	exit status 1
	%

Removing this check allows 'go doc sort.interface' or 'go doc
types.type' to discover sort.Interface and go/types.Type.

Easily

Fixes #34656.

Change-Id: I84352e83dd7f91a232f45a44d1a52f019a1a9a06
Reviewed-on: https://go-review.googlesource.com/c/go/+/205778Reviewed-by: Caleb Spare <cespare@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>

src/cmd/doc/main.go

@@ -358,17 +358,11 @@ func parseSymbol(str string) (symbol, method string) {
 	case 1:
 	case 2:
 		method = elem[1]
-		if !token.IsIdentifier(method) {
-			log.Fatalf("invalid identifier %q", method)
-		}
 	default:
 		log.Printf("too many periods in symbol specification")
 		usage()
 	}
 	symbol = elem[0]
-	if !token.IsIdentifier(symbol) {
-		log.Fatalf("invalid identifier %q", symbol)
-	}
 	return
 }