Revert "riscv: Clear pending interrupts before enabling IPIs"

Revert "riscv: Clear pending interrupts before enabling IPIs"

Clearing MIP.MSIP is not guaranteed to do anything by the spec. In
addition, most existing RISC-V hardware does nothing when this bit is set.

The following commits "riscv: Use a valid bit to ignore already-pending
IPIs" and "riscv: Clear pending IPIs on initialization" should implement
the original intent of the reverted commit in a more robust manner.

This reverts commit 9472630337e7c4ac442066b5a752aaa8c3b4d4a6.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>