tests: drivers: flash: fix build error

delete already defined storage_partition node in board DTS, in order to
be able to build test with included overlay

Signed-off-by: Abderrahmane JARMOUNI <[email protected]>

Author: JarmouniA

tests/drivers/flash/stm32/boards/stm32h750b_dk.overlay

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/delete-node/ &storage_partition;
+
 &flash0 {
 	partitions {
 		compatible = "fixed-partitions";