Cleanup: usages of bt_value_array_borrow_element_by_index{,_const}()

Cleanup: usages of bt_value_array_borrow_element_by_index{,_const}()

Checking the return value of this function against NULL is mostly
useless because in case of an out-of-bound access the function would
return a pointer value passed the end of the array (junk).

This commit removes NULL checks and `BT_ASSERT()`on the return value of
this function.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ifea5772d6b9f61487ef7295763f8f8929649b125
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2253
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>