Removes main from generated code

#### Summary:

This removes the ability to generate a main function for a generated model from within Stan. This is still available from CmdStan.

#### Intended Effect:

Options to generate an `int main` function has been removed from the generator.

#### How to Verify:

Look at generated C++ code.

#### Side Effects:

No. This functionality has already been moved to CmdStan.

This may affect RStan and PyStan by having one less argument to call in src/stan/gm. This should be a fairly easy fix from those interfaces.

#### Documentation:

This was an undocumented 'feature' of Stan that was removed.

#### Reviewer Suggestions:

Anyone.