Remove support for x-goog-emulator-instructions

[coryan]

Most likely, this is used only by the C++ client library, and the implementation is pretty ugly. The Cloud C++ team is planning to change their tests to use x-test-id. After that happens and we confirm that the other libraries do not use it either, we can remove the code.

The following libraries are known not to use it:

• Java
• Go
• Python
• Node
• Ruby
• PHP
• C#
• C++