argon2: Don't call _endthreadex/pthread_exit

Returning from the thread creation function is documented to be a valid
way of exiting a thread on both Windows and pthread systems. Removing
the explicit call avoids the need to install libgcc_s.so in initramfs
for glibc systems, and slightly reduces code size.

Upstream: https://github.com/P-H-C/phc-winner-argon2/pull/331

lib/crypto_backend/argon2/thread.c

@@ -46,12 +46,4 @@ int argon2_thread_join(argon2_thread_handle_t handle) {
 #endif
 }
 
-void argon2_thread_exit(void) {
-#if defined(_WIN32)
-    _endthreadex(0);
-#else
-    pthread_exit(NULL);
-#endif
-}
-
 #endif /* ARGON2_NO_THREADS */