correct removal of old files with `--force`

`rm` would fail when one of the files is not present, which is quite
undesirable log (not harmful until `set -e` is introduced).

target/bin/rspamd-dkim

@@ -177,7 +177,9 @@ function _create_keys() {
       exit 1
     else
       _log 'info' "Overwriting existing files as the '--force' option was supplied"
-      rm "${PUBLIC_KEY_FILE}" "${PUBLIC_KEY_DNS_FILE}" "${PRIVATE_KEY_FILE}"
+      [[ -f ${PUBLIC_KEY_FILE} ]] && rm "${PUBLIC_KEY_FILE}"
+      [[ -f ${PUBLIC_KEY_DNS_FILE} ]] && rm "${PUBLIC_KEY_DNS_FILE}"
+      [[ -f ${PRIVATE_KEY_FILE} ]] && rm "${PRIVATE_KEY_FILE}"
     fi
   fi