perf: Add comment about current state of PERF_MEM_LVL_* namespace and remove an extra line

Add a comment about PERF_MEM_LVL_* namespace being depricated
to some extent in favour of added PERF_MEM_{LVLNUM_,REMOTE_,SNOOPX_}
fields.

Remove an extra line present in perf_mem__lvl_scnprintf function.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20211006140654.298352-2-kjain@linux.ibm.com