Remove duplicate abstraction

In https://gitlab.mpi-sws.org/FP/iris-coq/blob/master/theories/proofmode/class_instances_bi.v#L13, with respect to the following three Global Instance declarations.

PROP is already abstracted as a section variable, and the extra generality is not used.

Removing {PROP : bi} here is needed for compatibility with coq/coq#8820.