Remove variable only used for an assert

When compiled with -DNDEBUG, we get warnings about set-but-unused variables.
In general, it's not something we care about, but since removing those
variables arguably makes the code nicer, let's just to it in this case.

src/basic/gcrypt-util.c

@@ -7,12 +7,10 @@
 #include "hexdecoct.h"
 
 void initialize_libgcrypt(bool secmem) {
-        const char *p;
         if (gcry_control(GCRYCTL_INITIALIZATION_FINISHED_P))
                 return;
 
-        p = gcry_check_version("1.4.5");
-        assert(p);
+        assert_se(gcry_check_version("1.4.5"));
 
         /* Turn off "secmem". Clients which wish to make use of this
          * feature should initialize the library manually */