cobol: Replace quadratic loop removing std::set elements

cobol: Replace quadratic loop removing std::set elements

There's no need to keep using std::find_if from the beginning of the
container after every removal, just update the iterator after erasing an
element.

This is how C++20 std::erase_if is implemented.

gcc/cobol/ChangeLog:

	* except.cc (cbl_enabled_exceptions_t::turn_on_off): Replace
	quadratic loop with a single pass.