[REF] Minor code cleanup on the setting of contact greetings.

I'm trying to review https://github.com/civicrm/civicrm-core/pull/15725 but through no fault of that PR it hits
on one of the parts of the Export code that I still find unreadable :-( This is a further readability fix & it
starts to point me to how to get past the underlying WTF about this code.

Note that I removed the IF around sharedAddress. From Monish's PR I found out that we have a
fairly long-standing regression where the sharedAddress code doesn't actually work :-( so
I'm comfortable this won't break anything :-). We also have good test cover on this chunk from
probably around when the shared address part got broken....

Looking in the UI there is no implication that the greeting for a shared address would display differently -
which is the only explanation I can think of for different handling here. Of course until we
remove the later IF the shared address would do things differently - erm if it worked.

ALl of which is a long way of saying the removal of the IF won't change anything