Remove a number of checks for functions/headers that are always present

Remove a number of checks for functions/headers that are always present

For the unlink function we actually have code that just ignores
the unlink call if the unlink function is not present. But all
platforms should have an unlink function.

This also removes all conditionals check for the headers that
belong to the C99 standard library header list
(https://en.cppreference.com/w/c/header).

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210406162518.4075-3-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22053.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>