lib: sbi: Do not clear active_events for cycle/instret when stopping

Those events are enabled by default and should not be reset afterwards
since when using SBI_PMU_CFG_FLAG_SKIP_MATCH, it leads to unaccessible
counters after the first use.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>