chore(CloudCommonProtos): remove orphaned AttributeContext metadata

[sofisl]

The AttributeContext proto correctly belongs to CommonProtos (which maps to google/rpc), not CloudCommonProtos (which maps to google/cloud/audit, etc.). During the migration of common-protos to this repository in PR #7554, the metadata file was accidentally duplicated here, but the actual source classes were correctly placed in CommonProtos.

This orphaned metadata file creates a namespace collision when both packages are loaded, as shown in the test below:

<?php
require __DIR__ . '/vendor/autoload.php';

if (class_exists('\GPBMetadata\Google\Rpc\Context\AttributeContext')) {
    echo "Class exists\n";
    $reflector = new \ReflectionClass('\GPBMetadata\Google\Rpc\Context\AttributeContext');
    echo "Found in: " . $reflector->getFileName() . "\n";
} else {
    echo "Class not found\n";
}

Output on main when testing in CloudCommonProtos:

Class exists
Found in: /usr/local/google/home/sofialeon/google-cloud-php/CloudCommonProtos/metadata/Rpc/Context/AttributeContext.php

However, CloudCommonProtos lacks the src class (Google\Rpc\Context\AttributeContext), which relies entirely on CommonProtos. Removing this duplicate file ensures the metadata is correctly loaded from CommonProtos, resolving the shadow collision. Librarian correctly flagged this duplicate file for deletion.

BREAKING_CHANGE_REASON=Removing legacy orphaned proto files that were left behind by OwlBot.

For googleapis/librarian#7366