As libpldm is being migrated to openbmc/libpldm, the documentation of
libpldm is also migrated, so removing it from pldm repository.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: Iafe87e6e0ccea56ff92843c4c0130b120e81e7d0