Remove `linear` parameter from `lax.cond_p`

[dfm]

As far as I can tell, it seems like the linear parameter in the lax.cond_p primitive only exists for historical reasons. It could be used for type checking in _cond_transpose, but that was removed because of #14026. With this in mind, we could stop tracking this parameter as implemented in this PR, unless we expect that we'd want to re-introduce the type checking in the future.