The current perf implementation in RISC-V is not very useful as it can not
count any events other than cycle/instructions. Moreover, perf record
can not be used or the events can not be started or stopped.

Remove the implementation now for a better platform driver in future
that will implement most of the missing functionality.
Reviewed-by: NAnup Patel <anup@brainfault.org>
Signed-off-by: NAtish Patra <atish.patra@wdc.com>
Signed-off-by: NAtish Patra <atishp@rivosinc.com>
Signed-off-by: NPalmer Dabbelt <palmer@rivosinc.com>