remove stale comments and unused constant

Author: BigTailWolf

google/internal/externalaccount/basecredentials.go

@@ -16,8 +16,6 @@ import (
 	"golang.org/x/oauth2/google/internal/stsexchange"
 )
 
-var maxUnixTime = time.Unix(1<<63-1, 999999999)
-
 // now aliases time.Now for testing
 var now = func() time.Time {
 	return time.Now().UTC()
@@ -245,8 +243,6 @@ func (ts tokenSource) Token() (*oauth2.Token, error) {
 	}
 
 	// The RFC8693 doesn't define the explicit 0 of "expires_in" field behavior.
-	// In practice a lot of sts request is using 0 means "never expire" for an sts token.
-	// So the logic here shall use a max unix time value.
 	if stsResp.ExpiresIn <= 0 {
 		return nil, fmt.Errorf("oauth2/google: got invalid expiry from security token service")
 	}