[REF] Remove unnecessary complexity on im export

[eileenmcnaughton]

Overview

This just removes a couple of lines that were already marked as being likely unhelpful. Because the impact of
these lines is tested the test needs tweaking too - but it only affects the name of the fields in the temp
table and in internal php keys

Before

Temp table column name and php key name non-standard for im & phone fields (but no impact seemingly for phone as the title is the same as the column)

After

Extra handling removed, standardised

Technical Details

This means the internal temp table columns are now 'im' (per the field name) not 'im_screen_name' per the field title. Likewise the keys. It is a simplification

Comments

Note the testExportIMData tests this within an inch of it's life

[civibot]

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[eileenmcnaughton]

I've added has-test because I added exhaustive tests on this last round of cleanup in here

[colemanw]

Ok I think we can trust the tests on this one.