input: remove unused FLB_INPUT_RETURN

[nokute78]

#3629 (comment)
Refer to the comment, I sent a patch to remove unused function FLB_INPUT_RETURN.

No one uses on current master.

taka@locals:~/git/fluent-bit$ find . -name "*.[ch]"|xargs grep FLB_INPUT_RETURN
taka@locals:~/git/fluent-bit$ 

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change

• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found

Documentation

• [N/A] Documentation required for this feature

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.