remove AWS-specfic default user agent

[AllanZhengYP]

Current default user agent string is AWS specific. So the customization
is moved to AWS JS SDK.

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[kstich]

Should this be set to a value that's just not AWS-specific instead of being removed altogether? What's the resulting behavior when not set in the configuration at all?

[AllanZhengYP]

@kstich

What's the resulting behavior when not set in the configuration at all?

There won't be a user agent header in the request. The default user agent removed in this change is only the dependency. Currently the components for injecting user agent are scatterred across 2 repos. The middleware that injecting the user agent to the HTTP request lives in the AWS-specific customization. We should either move all the user agent customization to the AWS customization or move the middleware customization to this repo. I think the previous option makes more sense because: (1)It assumes HTTP requests; (2)It depends on servicdId and other AWS-specific configs