keys: drop shadowing dead prototype

The global variable pkcs7 does not exist.
Drop the variable declaration, but keep the struct prototype needed for
is_key_on_revocation_list().

Reported by clang:

    ./include/keys/system_keyring.h:104:67: warning: declaration shadows a variable in the global scope [-Wshadow]
      104 | static inline int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
          |                                                                   ^
    ./include/keys/system_keyring.h:76:30: note: previous declaration is here
       76 | extern struct pkcs7_message *pkcs7;
          |                              ^

Fixes: 56c58126 ("certs: Add EFI_CERT_X509_GUID support for dbx entries")
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>