module: Remove the unused function within

The function within is defined in the main.c file, but not called
elsewhere, so remove this unused function.

This routine became no longer used after commit ("module: replace
module_layout with module_memory").

kernel/module/main.c:3007:19: warning: unused function 'within'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4035


Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
[mcgrof: adjust commit log to explain why this change is needed]
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>