Remove recently unsupported userenv CURRENT_SQLn

[at-ng]

I realize this project is not actively maintained, but to whom it may concern:

If you recently started getting

ORA-02003: invalid USERENV parameter

it's likely because of this line

Logger/source/packages/logger.pkb

Line 373 in 600f0c4

append_ctx('CURRENT_SQLn');

only reached with l_detail_level in ('ALL'). If you need that, you might want to remove this line.