Put this in your init file:
(setq disabled-command-function 'ignore)
By default, disabling a command means that you get that prompt when you try to use it.
But you can override this design by setting variable
disabled-command-function to a function other than its default value,
You can see the code for disabling and enabling in library
novice.el. You can find this out for yourself by doing
C-h f disable-command and clicking the link to go to the file.
See also the Emacs manual, node Disabling.
nil value for property
disabled for symbol
mouse-save-then-kill means that that command is disabled. A
nil value means that that command is enabled (is not disabled). But your use of a non-
nil value does not mean that mouse right-clicking is disabled. It means only that command
mouse-save-then-kill is disabled. It so happens that a right-click invokes that command by default.