Removed custom user agent string from outgoing requests.

[tonyanziano]

Fixes #1248

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This reverts #1092 as the custom user agent does not apply to outgoing requests from the extensions, however, Electron automatically attaches one of the form BotFrameworkEmulator/{version} to all outgoing requests originating in the app.

[coveralls]

Pull Request Test Coverage Report for Build 2381

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 55.459%

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Totals
Change from base Build 2379:	0.01%
Covered Lines:	4894
Relevant Lines:	8317

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2362

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 55.42%

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Totals
Change from base Build 2348:	-0.001%
Covered Lines:	4892
Relevant Lines:	8318

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💛 - Coveralls