tests: test_auto_source_discovery_grouping: remove dir_sep variable

tests: test_auto_source_discovery_grouping: remove dir_sep variable

It isn't used.  Fixes the following shellcheck diagnostics:

    In test_auto_source_discovery_grouping line 42:
            dir_sep='\'
                     ^-- SC1003: Want to escape a single quote? echo 'This is how it'\''s done'.

    In test_auto_source_discovery_grouping line 44:
            dir_sep='/'
            ^-----^ SC2034: dir_sep appears unused. Verify use (or export if used externally).

Reported-by: shellcheck
Change-Id: I9db00870cd697d5ad3bd5aa362a030a4b75f4e4c
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2419
Tested-by: jenkins <jenkins@lttng.org>