refactor(sts): remove unused IsExpired() helper functions

The session_helpers.go file contained two unused IsExpired() methods:
- Credentials.IsExpired()
- SessionInfo.IsExpired()

These were never called anywhere in the codebase. The actual expiration
checks use:
- isCredentialExpired() in weed/s3api/auth_credentials.go (S3 auth)
- Direct time.Now().After() checks

Removing unused code improves code clarity and reduces maintenance burden.

weed/iam/sts/session_helpers.go

@@ -1,29 +0,0 @@
-package sts
-
-import "time"
-
-// IsExpired returns true if the credentials have expired
-func (c *Credentials) IsExpired() bool {
-	if c == nil {
-		return true
-	}
-	// Treat zero-time expiration as expired (uninitialized credentials)
-	// This prevents treating uninitialized credentials as valid
-	if c.Expiration.IsZero() {
-		return true
-	}
-	return time.Now().After(c.Expiration)
-}
-
-// IsExpired returns true if the session has expired
-func (s *SessionInfo) IsExpired() bool {
-	// If SessionInfo is nil, consider it expired
-	if s == nil {
-		return true
-	}
-	// Treat zero-time expiration as expired (uninitialized session)
-	if s.ExpiresAt.IsZero() {
-		return true
-	}
-	return time.Now().After(s.ExpiresAt)
-}