lwip: Remove `sys_mbox_trypost_fromisr` macro

Avoids multiple declarations of `sys_mbox_trypost`.
lwIP defines `sys_mbox_trypost_fromisr` in one
of its source files and the warning is generated
when that file is also included.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>