Bug 36796: (follow-up) Fix column descriptions for the statistics table

Bug 36796: (follow-up) Fix column descriptions for the statistics table

This follow-up to bug 32312 fixes the descriptions for the statistics
database columns, used in guided reports.

It removes the duplicate statistics section in Koha/Database/Columns.pm
so that descriptions are displayed for all columns, as originally
intended.

It also adds a description for the interface column (a new column added
after the original patch was pushed).

Test plan:
1. Go to Reports > Guided reports > Create guided report.
2. Choose 'Circulation' for the module to report on and select
   'Next'.
3. Choose 'Tabular' for the type of report and select 'Next'.
4. Scroll down to the statistics table section and note that some
   columns have descriptions and some don't:
     Statistics date and time / statistics.datetime
     Library / statistics.branch
     Value / statistics.value
     Type / statistics.type
     Item number / statistics.itemnumber
     Item type / statistics.itemtype
     statistics.other
     statistics.location
     Borrower number / statistics.borrowernumber
     statistics.ccode
     statistics.categorycode
     statistics.interface
5. Apply the patch.
6. Restart all the things (restart_all).
7. Repeat steps 1 to 4.
8. Scroll down to the statistics table section again and note that all
   columns now have descriptions:
     Statistics date and time / statistics.datetime
     Library / statistics.branch
     Value / statistics.value
     Transaction type / statistics.type
     SIP mode / statistics.other
     Item number / statistics.itemnumber
     Koha item type / statistics.itemtype
     Shelving location / statistics.location
     Borrower number / statistics.borrowernumber
     Collection / statistics.ccode
     Patron category / statistics.categorycode
     Interface / statistics.interface
9. Sign off :D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>