Remove separate directory for `gaussian_elimination_pivoting.py` (#11445)

* updating DIRECTORY.md

* Remove separate directory for gaussian_elimination_pivoting.py

Delete the directory linear_algebra/src/gaussian_elimination_pivoting/
and move its algorithm file, gaussian_elimination_pivoting.py, into the
parent src/ directory. The gaussian_elimination_pivoting/ directory only
exists because gaussian_elimination_pivoting.py reads an example numpy
array from matrix.txt, but this input file and IO operation is entirely
unnecessary because gaussian_elimination_pivoting.py already has the
exact same array hard-coded into a variable.

* updating DIRECTORY.md

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