Temporal.Duration.prototype.{round,total}: Unnecessary call to CreateTemporalDate

[anba]

Temporal.Duration.prototype.round, step 34.d:

Set plainRelativeTo to ! CreateTemporalDate(precalculatedPlainDateTime.[[ISOYear]], precalculatedPlainDateTime.[[ISOMonth]], precalculatedPlainDateTime.[[ISODay]], zonedRelativeTo.[[Calendar]]).

But plainRelativeTo isn't used in later steps (resp. it doesn't matter if it's still undefined or not).

Also applies to the similar steps in Temporal.Duration.prototype.total.

[ptomato]

I'd like to revisit this after it's clear whether we have agreement to remove custom calendars. I think this if-clause would go away in that case.

[ptomato]

Obsoleted by #2925.