Get rid of the separate serial_schedule list of tests.

Get rid of the separate serial_schedule list of tests.

Having to maintain two lists of regression test scripts has proven
annoyingly error-prone.  We can achieve the effect of the
serial_schedule by running the parallel_schedule with
"--max_connections=1"; so do that and remove serial_schedule.

This causes cosmetic differences in the progress output, but it
doesn't seem worth restructuring pg_regress to avoid that.

Discussion: https://postgr.es/m/899209.1620759506@sss.pgh.pa.us