remove lp__ argument from _lpmf functions

[nhuurre]

A quick fix for issue #335

See my comment on the issue for reasons why I chose this approach rather than the one suggested by the issue title.

Copyright holder: Niko Huurre

By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[seantalts]

I read the issue but I'm not sure what the approach here is - the legal argument you laid out on the issue might suggest we no longer allow all functions containing the characters lp to access lp, right? But the code here is all in the backend - I would have expected to see some changes to the front end to print pretty error messages when the user wants to do that but they aren't allowed to anymore. Could you write a few sentences explaining what the approach is? Thanks!

[seantalts]

Oh, I think I get it - the front-end was already doing the right thing, only the backend had the spec wrong for functions containing lp. So the stanc3 frontend is already rejecting the models that ask for access to lp in a user-defined distribution, but the backend was following the stan2 compiler in part of its generated code and trying to pass along lp__ to all distribution functions always.

Great fix, thank you!

[seantalts]

(PS I really enjoyed the name of this branch :P)