Rely on plan_cache_mode to force generic plans in partition_prune test.

This file had a very weird mix of tests that did "set plan_cache_mode =
force_generic_plan" to get a generic plan, and tests that relied on
using five dummy executions of a prepared statement.  Converting them
all to rely on plan_cache_mode is more consistent and shaves off a
noticeable fraction of the test script's runtime.

Discussion: https://postgr.es/m/11952.1569536725@sss.pgh.pa.us

Author: tglsfdc