Resolves: fdo#78150 Ctrl+Backspace does not erase first character

See https://bugs.freedesktop.org/show_bug.cgi?id=78150#c1
and https://bugs.freedesktop.org/show_bug.cgi?id=78150#c2
1) there's a wrong conversion with negative number given to a xub_StrLen parameter, anyway it works on 4.2
2) The existing code can be simplified since we don't need to remove spaces before the word to delete

Change-Id: I7e2510038666018059a54ac09c1a599154084b48
Reviewed-on: https://gerrit.libreoffice.org/9245
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>