ITS: Fix unused variable warning for read_size

Fix unused variable warning for read_size.

Variable is not used so we delete it.

Change-Id: I0a61834c852862e79fbf4654995bc08ac4ebb757
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
diff --git a/secure_fw/partitions/internal_trusted_storage/tfm_internal_trusted_storage.c b/secure_fw/partitions/internal_trusted_storage/tfm_internal_trusted_storage.c
index 6ac360c..691db53 100644
--- a/secure_fw/partitions/internal_trusted_storage/tfm_internal_trusted_storage.c
+++ b/secure_fw/partitions/internal_trusted_storage/tfm_internal_trusted_storage.c
@@ -571,11 +571,6 @@
 {
     psa_status_t status;
 
-    #if (PSA_FRAMEWORK_HAS_MM_IOVEC != 1) && defined(TFM_PARTITION_INTERNAL_TRUSTED_STORAGE) \
-        && (ITS_ENCRYPTION != 1)
-    size_t read_size;
-    #endif
-
 #ifdef TFM_PARTITION_TEST_PS
     /* The PS test partition can call tfm_its_get() through PS code. Treat it
      * as if it were PS.