DOCS/man/input.rst: remove input commands subject to change heading

Most of the commands list here are several years old and we will
probably never change them. And it is already stated in the description
of individual commands if they are deprecated or subject to change.

Furthermore, it is easy to add new commands at the end of this section
by accident. I added load-config-file and load-input-conf here without
realizing it's a separate section, and I assume this was also the case
for begin-vo-dragging.

DOCS/man/input.rst

@@ -970,10 +970,6 @@ Remember to quote string arguments in input.conf (see `Flat command syntax`_).
 ``context-menu``
     Show context menu on the video window. See `Context Menu`_ section for details.
 
-
-Input Commands that are Possibly Subject to Change
---------------------------------------------------
-
 ``af <operation> <value>``
     Change audio filter chain. See ``vf`` command.