Remove duplicate implementation of median of two arrays algorithm (#11420)

* Remove duplicate implementation of median of two arrays algorithm

Remove maths/median_of_two_arrays.py because the repo has two
implementations of this algorithm, with
data_structures/arrays/median_two_array.py being the other. Even though
maths/median_of_two_arrays.py is the older implementation, the newer
implementation is better documented, has better error handling, and is
already located in a more appropriate directory.

* updating DIRECTORY.md

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Co-authored-by: tianyizheng02 <tianyizheng02@users.noreply.github.com>