Remove unused field, triggering a compiler warning

[jmid]

This PR removes an unused field causing an annoying compiler warning:

$ dune build
File "src/core/QCheck2.ml", line 1172, characters 2-20:
1172 |   fun_gen: 'b Gen.t; (** How to generate output values *)
         ^^^^^^^^^^^^^^^^^^
Warning 69 [unused-field]: record field fun_gen is never read.
(However, this field is used to build or mutate values.)
File "src/core/QCheck2.ml", line 1172, characters 2-20:
1172 |   fun_gen: 'b Gen.t; (** How to generate output values *)
         ^^^^^^^^^^^^^^^^^^
Warning 69 [unused-field]: record field fun_gen is never read.
(However, this field is used to build or mutate values.)

Note: The removed code is not visible in the QCheck2 interface and the change is thereby invisible to end-users.
With this PR QCheck now compiles cleanly (silently) on 5.2.

Removing code without affecting features is a pleasure... 😄