Bug 13050: Follow-up for bug 12371

Bug 13050: Follow-up for bug 12371

This patch simplifies the SQL query in Letters.pm for table
borrower_modifications.
It also addresses the only case this query is used in opac-memberentry.
An unused variable in Letters.pm is removed.

Test plan:
Enable selfregistration on opac.
Set verification by email to required in prefs too.
Self-register two new users.
Check the email notices generated.
Verify the new users with the tokens in their notice.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Much cleaner SQL

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Cleaner and works as described, no regressions found.
Passes tests and QA script.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>