Remove unused chain behavior

[lucasfcosta]

Purpose (TL;DR)

This PR removes the old chain behavior method introduced in #1227 since we're not using it anymore.

This is also related to #817 and #1100.

Background (Problem in detail)

Since #1302, most of the old behaviors were moved to a file called defaultBehaviors and now we're using the addBehavior method to add behavior methods to the stub. This method assigns a new function to the stub's proto and this method both runs the behavior method and then returns this.stub or this if this.stub is falsy, which is exactly the same thing the chain behavior did.

Due to this fact, the chain method is not useful anymore because we're not using it internally anymore and no user needs to call it, since every behavior will return exactly the same value (the stub itself). Another point that backs up this assumption is that we have no tests that test chain specifically, which means this isn't something meant to be used by our end-users.

Solution

Given the explanation above, I simply removed the chain behavior and all tests still pass.

How to verify

1. Check out this branch (see github instructions below)
2. npm install
3. npm test -> All tests still pass

Let me know if I missed anything and I'll be more than happy to fix it. Thanks!

[coveralls]

Coverage decreased (-1.4%) to 93.984% when pulling 70e96f5 on lucasfcosta:remove-unused-chain-behavior into e0cfccc on sinonjs:master.

[coveralls]

Coverage decreased (-1.4%) to 93.984% when pulling 70e96f5 on lucasfcosta:remove-unused-chain-behavior into e0cfccc on sinonjs:master.

[mantoni]

Given the simplicity of this change, I think it could go into the still pending 2.0 release @fatso83 @mroderick @cjohansen?

[fatso83]

Absolutely, I deleted that code myself in another (unmerged) branch. Merging.