docs/zh_CN: riscv: Remove the translation of pmu.rst

Synchronous translation from the following:

[1]: commit 23b1f183("Documentation: riscv:
     Remove the old documentation")

[2]: commit 8933e7f2 ("Documentation: riscv: remove
     non-existent directory from table of contents")

Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/6fe45d69210300a6c065262470bce963f8dc0ec3.1654685338.git.zhoubinbin@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>