Deprecate quickstart.term_decode() and remove internal uses

Per the Python 3 docs, input() always returns a string:

https://docs.python.org/3/library/functions.html#input

> The function then reads a line from input, converts it to a
> string (stripping a trailing newline), and returns that.

The stubs from typeshed say the same:

5c69373890/stdlib/3/builtins.pyi (L835)

Here is the implementation from CPython with also shows a call to
PyUnicode_Decode on the result:

https://github.com/python/cpython/blob/3.7/Python/bltinmodule.c#L1960-L2143

As the value is always a string, there is nothing to decode. Therefore
the call to term_decode() unnecessary and can safely be removed.

With this in mind, must adjust quickstart tests to be more
representative.

doc/extdev/index.rst

@@ -152,6 +152,16 @@ The following is a list of deprecated interfaces.
      - 4.0
      - ``docutils.nodes.abbreviation``
 
+   * - ``sphinx.cmd.quickstart.term_decode()``
+     - 2.0
+     - 4.0
+     - N/A
+
+   * - ``sphinx.cmd.quickstart.TERM_ENCODING``
+     - 2.0
+     - 4.0
+     - ``sys.stdin.encoding``
+
    * - ``sphinx.config.check_unicode()``
      - 2.0
      - 4.0