Do not remove gnucash_core.c in clean rule

That file is a built source. It is already removed by the maintainer-clean
rule.

Author: Sébastien Villemot <sebastien@debian.org>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23302 57a11ea4-9604-0410-9ed3-97b8803252fd