Fix segfault in test-portfolios after debug build.

gnc-clear-current-session deletes the pricedb so deleting it first
is unnecessary. Deleting it first and not nulling the pointer in
the book leads to accessing freed memory, and debug mode poisons
that to force a crash when it happens.

gnucash/report/standard-reports/test/test-portfolios.scm

@@ -56,9 +56,6 @@
         (test-assert (format #f "wrong-option ~a ~a" section name) #f))))
 
 (define (teardown)
-  (gnc-pricedb-destroy
-   (gnc-pricedb-get-db
-    (gnc-get-current-book)))
   (gnc-clear-current-session))
 
 (define (null-test variant uuid)