chore(ConfidentialComputing): remove obsolete files

[sofisl]

This PR removes obsolete structural files for ConfidentialComputing that are no longer generated.

Testing Deletions

I attempted to load the deleted classes to prove they do not exist anymore.

Test Script:

<?php
require 'vendor/autoload.php';
echo "Testing deleted classes for ConfidentialComputing...\n";
$success = true;
if (class_exists('Google\Cloud\ConfidentialComputing\V1\VerifyAttestationRequest\AwsPrincipalTagsOptions') || interface_exists('Google\Cloud\ConfidentialComputing\V1\VerifyAttestationRequest\AwsPrincipalTagsOptions') || trait_exists('Google\Cloud\ConfidentialComputing\V1\VerifyAttestationRequest\AwsPrincipalTagsOptions')) {
    echo "ERROR: Google\Cloud\ConfidentialComputing\V1\VerifyAttestationRequest\AwsPrincipalTagsOptions still exists!\n";
    $success = false;
} else {
    echo "SUCCESS: Google\Cloud\ConfidentialComputing\V1\VerifyAttestationRequest\AwsPrincipalTagsOptions does not exist.\n";
}
if (!$success) exit(1);

Output:

Testing deleted classes for ConfidentialComputing...
SUCCESS: Google\Cloud\ConfidentialComputing\V1\VerifyAttestationRequest\AwsPrincipalTagsOptions does not exist.

BREAKING_CHANGE_REASON=In googleapis PR #8549 (September 2025), the message AwsPrincipalTagsOptions was moved to a top-level message. OwlBot correctly generated the new file but failed to delete the obsolete nested VerifyAttestationRequest\AwsPrincipalTagsOptions.php file.

For googleapis/librarian#7366