Remove dry-run mode from isolationtester

Remove dry-run mode from isolationtester

The original purpose of the dry-run mode is to be able to print all the
possible permutations from a spec file, but it has become less useful
since isolation tests have improved regarding deadlock detection as one
step not wanted by the author could block indefinitely now (originally
the step blocked would have been detected rather quickly).  Per
discussion, let's remove it.

This is a backpatch of 9903338 for 9.6~12.  It is proving to become
useful to have on those branches so as the code gets consistent across
all supported versions, as a matter of improving the output generated by
isolationtester.

Author: Michael Paquier
Reviewed-by: Asim Praveen, Melanie Plageman
Discussion: https://postgr.es/m/20190819080820.GG18166@paquier.xyz
Discussion: https://postgr.es/m/794820.1623872009@sss.pgh.pa.us
Backpatch-through: 9.6