[Interactive] Remove dependency of ENV_ADDITIONAL_USER_AGENT

[jiasli]

Fix #1480: az interactive crashes with "cannot import name 'ENV_ADDITIONAL_USER_AGENT'"

This issue is introduced by Azure/azure-cli#12734 which removed the redundant logic of ENV_ADDITIONAL_USER_AGENT, because it is already taken care of by msrest.

This PR removes the dependency of AZURE_HTTP_USER_AGENT and hard-codes it because msrest doesn't expose it:

https://github.com/Azure/msrest-for-python/blob/4cc8bc84e96036f03b34716466230fb257e27b36/msrest/pipeline/universal.py#L70

class UserAgentPolicy(SansIOHTTPPolicy):
    _USERAGENT = "User-Agent"
    _ENV_ADDITIONAL_USER_AGENT = 'AZURE_HTTP_USER_AGENT'

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[yonzhan]

add to S168