check_radius: Remove unused variable

The radcli library doesn't define the ENV type, and we don't use it
anyway.

plugins/check_radius.c

@@ -92,7 +92,6 @@ char *config_file = NULL;
 unsigned short port = PW_AUTH_UDP_PORT;
 int retries = 1;
 int verbose = FALSE;
-ENV *env = NULL;
 
 /******************************************************************************