The post_msg_page was introduced in 2014 in
commit b29ef354 ("Drivers: hv: vmbus: Cleanup hv_post_message()")

Commit 68bb7bfb ("X86/Hyper-V: Enable IPI enlightenments") introduced
the hyperv_pcpu_input_arg in 2018, which can be used in hv_post_message().

Remove post_msg_page to simplify the code a little bit.
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Jinank Jain <jinankjain@linux.microsoft.com>
Link: https://lore.kernel.org/r/20230408213441.15472-1-decui@microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>