This reverts commit d4e2a843, which added a new user with limited
permissions to run the TAP tests of pg_rewind.  Buildfarm machine
members on Windows jacana and bowerbird have been complaining about
that, the new role not being able to run the rewind because SSPI is not
configured to allow it.

Fixing the test requires passing down directly the new user to
pg_regress with --create-role so as SSPI can work properly.

Reported-by: Andrew Dunstan
Discussion: https://postgr.es/m/3cd43d33-f415-cc41-ade3-7230ab15b2c9@2ndQuadrant.com