tests: drop calls to unsetenv SYSTEMD_MEMPOOL

Coverity was complaining that we don't check the return value, which we stopped
doing in 772e0a76f3.

But it seems that we don't want those calls at all. The test was originally
added with the call in a6ee01caf3, but I don't
see why we should override this. If the user wants to execute the test with
mempool disabled, we shouldn't ignore that.

Coverity CID#1444464, CID#1444466.

src/libsystemd/sd-device/test-sd-device-thread.c

@@ -30,8 +30,6 @@ int main(int argc, char *argv[]) {
         const char *key, *value;
         int r;
 
-        unsetenv("SYSTEMD_MEMPOOL");
-
         r = sd_device_new_from_syspath(&loopback, "/sys/class/net/lo");
         if (r < 0)
                 return handle_error_errno(r, "Failed to create loopback device object");

src/libudev/test-udev-device-thread.c

@@ -28,8 +28,6 @@ int main(int argc, char *argv[]) {
         pthread_t t;
         int r;
 
-        unsetenv("SYSTEMD_MEMPOOL");
-
         loopback = udev_device_new_from_syspath(NULL, "/sys/class/net/lo");
         if (!loopback)
                 return handle_error_errno(errno, "Failed to create loopback device object");