refactor: remove requirements/edx-sandbox/shared.[in,txt]

These files were used to assist the Python 3.5 -> 3.8 upgrade,
but they are no longer needed nor referened anywhere. They haven't
been updated for years.

requirements/edx-sandbox/shared.in

@@ -1,14 +0,0 @@
-# Core dependencies shared between Python sandboxes for secured execution and edx-platform.
-#
-# DON'T JUST ADD NEW DEPENDENCIES!!!
-#
-# If you open a pull request that adds a new dependency, you should:
-#   * verify that the dependency has a license compatible with AGPLv3
-#   * confirm that it has no system requirements beyond what we already install
-#   * run "make upgrade" to update the detailed requirements files
-
--c ../constraints.txt
-
-cryptography                        # Implementations of assorted cryptography algorithms
-lxml                                # XML parser
-nltk                                # Natural language processing; used by the chem package

requirements/edx-sandbox/shared.txt

@@ -1,34 +0,0 @@
-#
-# This file is autogenerated by pip-compile
-# To update, run:
-#
-#    make upgrade
-#
-cffi==1.14.5
-    # via cryptography
-click==7.1.2
-    # via
-    #   -c requirements/edx-sandbox/../constraints.txt
-    #   nltk
-cryptography==3.2.1
-    # via
-    #   -c requirements/edx-sandbox/../constraints.txt
-    #   -r requirements/edx-sandbox/shared.in
-joblib==0.14.1
-    # via
-    #   -c requirements/edx-sandbox/../constraints.txt
-    #   nltk
-lxml==4.5.0
-    # via
-    #   -c requirements/edx-sandbox/../constraints.txt
-    #   -r requirements/edx-sandbox/shared.in
-nltk==3.6.2
-    # via -r requirements/edx-sandbox/shared.in
-pycparser==2.20
-    # via cffi
-regex==2021.4.4
-    # via nltk
-six==1.16.0
-    # via cryptography
-tqdm==4.61.0
-    # via nltk