af_unix: Remove module remnants.

Since commit 97154bcf ("af_unix: Kconfig: make CONFIG_UNIX bool"),
af_unix.c is no longer built as module.

Let's remove unnecessary #if condition, exitcall, and module macros.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20231026212305.45545-1-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>