tests/shell: Fix shellcheck issues in tests/shell/stat+shadow_stat.sh tetscase

Running shellcheck on stat+shadow_stat.sh generates below
warning

In tests/shell/stat+csv_summary.sh line 26:
while read _num _event _run _pct
^--^ SC2034: _num appears unused. Verify use (or export if used externally).
^----^ SC2034: _event appears unused. Verify use (or export if used externally).
^--^ SC2034: _run appears unused. Verify use (or export if used externally).
^--^ SC2034: _pct appears unused. Verify use (or export if used externally).

This variable is intentionally unused since it is
needed to parse through the output. commit used "_"
as a prefix for this throw away variable. But this
stil shows warning with shellcheck v0.6. Fix this
by only using "_" instead of prefix and variable name.

Signed-off-by: Athira Rajeev <[e-mail]>
Tested-by: Ian Rogers <[e-mail]>
Reviewed-by: Kajol Jain <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/tests/shell/stat+csv_summary.sh

···
 #
 perf stat -e cycles  -x' ' -I1000 --interval-count 1 --summary 2>&1 | \
 grep -e summary | \
-while read summary _num _event _run _pct
+while read summary _ _ _ _
 do
 	if [ $summary != "summary" ]; then
 		exit 1
···
 #
 perf stat -e cycles  -x' ' -I1000 --interval-count 1 --summary --no-csv-summary 2>&1 | \
 grep -e summary | \
-while read _num _event _run _pct
+while read _ _ _ _
 do
 	exit 1
 done

tools/perf/tests/shell/stat+shadow_stat.sh

···
 {
 	perf stat -a --no-big-num -e cycles,instructions sleep 1  2>&1 | \
 	grep -e cycles -e instructions | \
-	while read num evt _hash ipc rest
+	while read num evt _ ipc rest
 	do
 		# skip not counted events
 		if [ "$num" = "<not" ]; then
···
 {
 	perf stat -a -A --no-big-num -e cycles,instructions sleep 1  2>&1 | \
 	grep ^CPU | \
-	while read cpu num evt _hash ipc rest
+	while read cpu num evt _ ipc rest
 	do
 		# skip not counted events
 		if [ "$num" = "<not" ]; then