Remove redundancy in Statistics labels

These labels were too long, obtrusive, and state the obvious.
We can make them shorter and friendlier by removing the “Number of”
nuisance. The equivalent dialog in Office 2013 also lacks it.
Brevity is a Good Thing™ in UIs.

Change-Id: Ia44f02cbcbba22a3161cac206511de4fe4daedcc
Reviewed-on: https://gerrit.libreoffice.org/11222
Reviewed-by: Joren De Cuyper <jorendc@libreoffice.org>
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>