Remove function that marks encr keys invalid

Just use existing zeroize function, which also takes care of zeroing out
the encryption key data.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index 95bf878..15cefb3 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -2284,7 +2284,7 @@
          * another images). Therefore, mark them as invalid to force their reload
          * by boot_enc_load().
          */
-        boot_enc_mark_keys_invalid(state->enc);
+        boot_enc_zeroize(state->enc);
 #endif
 
         image_index = BOOT_CURR_IMG(state);
@@ -2330,7 +2330,7 @@
          * another images). Therefore, mark them as invalid to force their reload
          * by boot_enc_load().
          */
-        boot_enc_mark_keys_invalid(state->enc);
+        boot_enc_zeroize(state->enc);
 #endif /* MCUBOOT_ENC_IMAGES */
 
         /* Indicate that swap is not aborted */