Remove Equals(), GetHashCode(), and ToString() methods from TextanalyticsClient

[maririos]

See https://github.com/Azure/azure-sdk-for-net/blob/master/sdk/textanalytics/Azure.AI.TextAnalytics/src/TextAnalyticsClient_private.cs#L117

[maririos]

After disusing it with the team, the purpose of having the methods is to hide them from Intellisense as System.Object contains them.
No action here, so closing