injection_points: Remove portions related to custom pgstats

injection_points: Remove portions related to custom pgstats

The test module injection_points has been used as a landing spot to
provide coverage for the custom pgstats APIs, for both fixed-sized and
variable-sized stats kinds.  Some recent work related to pgstats is
proving that this structure makes the implementation of new tests
harder.

This commit removes the code related to pgstats from injection_points,
and an equivalent will be reintroduced as a separate test module in a
follow-up commit.  This removal is done in its own commit for clarity.

Using injection_points for this test coverage was perhaps not the best
way to design things, but this was good enough while working on the
first flavor of the custom pgstats APIs.  Using a new test module will
make easier the introduction of new tests, and we will not need to worry
about the impact of new changes related to custom pgstats could have
with the internals of injection_points.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0sJgO6GAwgFxmzg9MVP=rM7Us8KKcWpuqxe-f5qxmpE0g@mail.gmail.com