Documentation: riscv: Remove the old documentation

Documentation: riscv: Remove the old documentation

The existing pmu documentation describes the limitation of perf
infrastructure in RISC-V ISA and limited feature set of perf in RISC-V.

However, SBI PMU extension and sscofpmf extension(ISA extension) allows to
implement most of the required features of perf. Remove the old
documentation which is not accurate anymore.

Reviewed-by: Anup Patel <[email protected]>
Signed-off-by: Atish Patra <[email protected]>
Signed-off-by: Atish Patra <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>