Added BotState.delete() method and tests

[Stevenic]

Fixes microsoft/botframework-sdk#5175

Description

Adds a new BotState.delete() method.

Specific Changes

Added method and unit tests.

Testing

Included

[coveralls]

Pull Request Test Coverage Report for Build 2366

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 82.223%

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Totals
Change from base Build 2364:	0.06%
Covered Lines:	2635
Relevant Lines:	3085

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💛 - Coveralls

[benbrown]

Is it worth updating the comment on the clear() method and/or the delete() method to specify the difference?

[Stevenic]

I put the differences in the remarks of each method but if you feel like you have better descriptions feel free to change.