perf tests: Fix shellcheck warning in stat_all_metricgroups

perf tests: Fix shellcheck warning in stat_all_metricgroups

Running shellcheck on stat_all_metricgroups.sh reports
below warning:

 In ./tests/shell/stat_all_metricgroups.sh line 7:
 function ParanoidAndNotRoot()
 ^-- SC2112: 'function' keyword is non-standard. Delete it.

As per the format, "function" is a non-standard keyword that
can be used to declare functions. Fix this by removing the
"function" keyword from ParanoidAndNotRoot function

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: 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/20231013073021.99794-4-atrajeev@linux.vnet.ibm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>