Drop useless Abort choice in USB backup selection dialog

[jsmeix]

An "Abort" choice in any 'select' dialog is useless
because simple [Ctrl]+[C] also aborts ReaR.

Cf. #1166

With that change recovery with backup on USB
and aborting at the USB backup selection dialog
works as follows:

RESCUE e205:~ # rear -d -D recover
...
Backup archive /tmp/rear.JAM8egDNTgZlexj/outputfs/rear/e205/20170117.1311/backup.tar.gz detected.
Backup archive /tmp/rear.JAM8egDNTgZlexj/outputfs/rear/e205/20170117.1324/backup.tar.gz detected.
Backup archive /tmp/rear.JAM8egDNTgZlexj/outputfs/rear/e205/20170117.1335/backup.tar.gz detected.
Select a backup archive.
1) 20170117.1311
2) 20170117.1324
3) 20170117.1335
#? 4
Invalid choice 4, try again (or press [Ctrl]+[C] to abort).
#? ^C
2017-01-18 09:22:29.979923804 Running exit tasks.
2017-01-18 09:22:29.982992270 Exit task 'umount -f -v '/tmp/rear.JAM8egDNTgZlexj/outputfs' >&2'
umount: /tmp/rear.JAM8egDNTgZlexj/outputfs (/dev/sdb1) unmounted
2017-01-18 09:22:29.994368475 Exit task 'rmdir -v /tmp/rear.JAM8egDNTgZlexj/outputfs >&2'
rmdir: removing directory, '/tmp/rear.JAM8egDNTgZlexj/outputfs'
2017-01-18 09:22:29.997847901 Exit task 'cleanup_build_area_and_end_program'
2017-01-18 09:22:30.000025643 Finished in 8 seconds
2017-01-18 09:22:30.002067549 Removing build area /tmp/rear.JAM8egDNTgZlexj
removed directory '/tmp/rear.JAM8egDNTgZlexj'
2017-01-18 09:22:30.011102008 End of program reached
2017-01-18 09:22:30.013068370 Exit task 'exec 8>&-'
2017-01-18 09:22:30.015225968 Exit task 'exec 7>&-'
2017-01-18 09:22:30.017316532 Exit task ''

RESCUE e205:~ # echo $?
130

When there are no objections I will "just merge" it.

[jsmeix]

Only FYI:
Same kind of backup selection dialog is also in
verify/RBME/default/540_choose_backup.sh
but I won't touch that because I know nothing at all about
the RBME (Rsync Backup Made Easy) backup method.