Clean up Mozl10n API in FTU #1058985

In the original code in line 272 we assign 'simPinLocked' entity to simCarried node. In line 273 we clear the translation of simNumber node.

So what I believe you want to do here is:
 - removeAttribute('data-l10n-id') and set textContent to null for simNumber node
 - set 'simPinLocked' data-l10n-id for simCarried node

Does it sound good?