Remove isolation test reindex-partitions

 Remove isolation test reindex-partitions
 
 The isolation test added by a6642b3 is proving to be unstable, as once
 the first transaction holding a lock on the top-most partitioned table
 or on a partition commits, the commit order of the follow-up DROP TABLE
 and REINDEX could become reversed depending on the timing.
 
 The only part of the test that could be entirely reliable is the one
 using a SHARE lock, allowing REINDEX to commit first, but it is the
 least interesting of the set.
 
 Per buildfarm members rorqual and mylodon.
 
 Discussion: https://postgr.es/m/E1kFSBj-00062c-Mu@gemulon.postgresql.org