test,modem-helpers: remove conditional compilation

We require already GLib 2.30

src/tests/test-modem-helpers.c

@@ -1296,13 +1296,7 @@ _mm_log (const char *loc,
     /* Dummy log function */
 }
 
-#if GLIB_CHECK_VERSION(2,25,12)
-typedef GTestFixtureFunc TCFunc;
-#else
-typedef void (*TCFunc)(void);
-#endif
-
-#define TESTCASE(t, d) g_test_create_case (#t, 0, d, NULL, (TCFunc) t, NULL)
+#define TESTCASE(t, d) g_test_create_case (#t, 0, d, NULL, (GTestFixtureFunc) t, NULL)
 
 int main (int argc, char **argv)
 {