Bug 10643: fix inappropriate uses of $sth->finish() in C4::ClassSource.pm

Bug 10643: fix inappropriate uses of $sth->finish() in C4::ClassSource.pm

This patch gets rid of finish() and replace prepare_cached by prepare.

From the man page

finish()
Indicate that no more data will be fetched from this statement handle
before it is either executed again or destroyed.
You almost certainly do not need to call this method.

Adding calls to "finish" after loop that fetches all rows is a common
mistake, don't do it, it can mask genuine problems like uncaught fetch errors.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>