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

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 <atrajeev@linux.vnet.ibm.com>
Tested-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kajol Jain <kjain@linux.ibm.com>
Cc: maddy@linux.ibm.com
Cc: disgoel@linux.vnet.ibm.com
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230907171540.36736-3-atrajeev@linux.vnet.ibm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>