Delete deadcode in VSTestBridge

[Youssef1313]

ExecuteRequestAsync(ExecuteRequestContext) is ONLY ever called by core MTP. Core MTP never passes VSTestDiscoverTestExecutionRequest or VSTestRunTestExecutionRequest.

The flow currently goes like:

• MTP calls ExecuteRequestAsync(ExecuteRequestContext), where the request is either DiscoverTestExecutionRequest or RunTestExecutionRequest
• In the bridge, we always get to the TestExecutionRequest branch of the switch, and call ExecuteRequestAsync(TestExecutionRequest,IMessageBus,CancellationToken).
• ExecuteRequestAsync(TestExecutionRequest,IMessageBus,CancellationToken) is abstract and is overridden by SynchronizedSingleSessionVSTestBridgedTestFramework. The implementation of the override handles DiscoverTestExecutionRequest and RunTestExecutionRequest by transforming them to VSTestDiscoverTestExecutionRequest and VSTestRunTestExecutionRequest and then calls either SynchronizedDiscoverTestsAsync or SynchronizedRunTestsAsync
• SynchronizedDiscoverTestsAsync and SynchronizedRunTestsAsync are abstract and are implementable by frameworks outside the bridge (e.g, MSTest, NUnit, Expecto). Those methods take the concrete VSTestDiscoverTestExecutionRequest and VSTestRunTestExecutionRequest as parameters.

Additional cleanup that can be done but is breaking change:

• VSTestDiscoverTestExecutionRequest and VSTestRunTestExecutionRequest don't need to inherit DiscoverTestExecutionRequest and RunTestExecutionRequest. In fact, I tend to think that MTP should have made DiscoverTestExecutionRequest and RunTestExecutionRequest sealed.
• The methods DiscoverTestsAsync and RunTestsAsync implemented in SynchronizedSingleSessionVSTestBridgedTestFramework are dead and are never called.