perf mem: Clean up perf_pmus__num_mem_pmus()

perf mem: Clean up perf_pmus__num_mem_pmus()

The number of mem PMUs can be calculated by searching the
perf_pmus__scan_mem().

Remove the ARCH specific perf_pmus__num_mem_pmus()

Tested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: ravi.bangoria@amd.com
Cc: james.clark@arm.com
Cc: will@kernel.org
Cc: mike.leach@linaro.org
Cc: renyu.zj@linux.alibaba.com
Cc: yuhaixin.yhx@linux.alibaba.com
Cc: tmricht@linux.ibm.com
Cc: atrajeev@linux.vnet.ibm.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: john.g.garry@oracle.com
Link: https://lore.kernel.org/r/20240123185036.3461837-8-kan.liang@linux.intel.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>