ci: Don't specify amount of memory

ci: Don't specify amount of memory

The number of CPUs is the cost-determining factor. Most instance types that
run tests have more memory/core than what we specified, there's no real
benefit in wasting that.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/20230808021541.7lbzdefvma7qmn3w@awork3.anarazel.de
Backpatch: 15-, where CI support was added