static_call: Remove __DEFINE_STATIC_CALL macro

Only DEFINE_STATIC_CALL use __DEFINE_STATIC_CALL macro now when
CONFIG_HAVE_STATIC_CALL is selected.

Only keep __DEFINE_STATIC_CALL() for the generic fallback, and
also use it to implement DEFINE_STATIC_CALL_NULL() in that case.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/329074f92d96e3220ebe15da7bbe2779beee31eb.1647253456.git.christophe.leroy@csgroup.eu