pg_upgrade: Fix some minor code issues
96ef3b8ff1cf1950e897fd2f766d4bd9ef0d5d56 accidentally copied a not
applicable comment from the float8_pass_by_value code to the
data_checksums code. Remove that.
87d3b35a1ca31a9d947a8f919a6006679216dff0 changed pg_upgrade to
checking the checksum version rather than just the Boolean presence of
checksums, but didn't change the field type in its ControlData struct
from bool. So this would not work correctly if there ever is a
checksum version larger than 1.