Remove C version of substitute-command-keys

Remove C version of substitute-command-keys

* src/doc.c (Fsubstitute_command_keys_old): Remove.
(syms_of_doc): Remove defsubr for Fsubstitute_command_keys_old.
* src/keymap.c (describe_map, describe_map_tree)
(describe_map_compare, describe_map_elt): Remove.
* src/keymap.h: Remove 'describe_map_tree'.
* test/lisp/help-tests.el (with-substitute-command-keys-test)
(help-tests-substitute-command-keys/compare)
(help-tests-substitute-command-keys/compare-all):
Don't test the C version of 'substitute-command-keys' removed
above.