By cleaning up the code the compiler does not emit the following warning
anymore:

    src/keystore-blob.c: In function 'keystore_get_secret':
    src/keystore-blob.c:25:15: warning: unused variable 'modifier' [-Wunused-variable]
      char *blob, *modifier, *payload;
                   ^~~~~~~~

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>