Remove `linear` parameter from `lax.cond_p`.
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.
