Fix regression with wall stuck flipping behavior exactly reversed

The reason why the wall stuck flipping behavior happened in the first
place was because the code went like this:

    if (jumppressed)
    {
        if (onground && gravitycontrol == 0)
        {
            gravitycontrol = 1;
        }
        if (onroof && gravitycontrol == 1)
        {
            gravitycontrol = 0;
        }
    }

Basically, if you were both on ground and on a roof (i.e. stuck in a
wall), you would flip, but then due to code order and the fact that the
statement is not connected to the previous one, you would immediately
unflip afterwards. But if you were already flipped then the only path
that can be taken is to unflip you, since it's the statement that
appears last.

52fceb3f69 replaces the onground/onroof
conditionals with any_onground/any_onroof, so any player entity would
allow you to flip. But otherwise the code is the same. So is that the
problem?

No; tracing it through with GDB reveals that when you flip,
gravitycontrol is being set to 1, but never being set to 0. And it turns
out that's because any_onroof is not getting set. And that happens
because of another thing that 52fceb3f69
did - which was to set any_onground/any_onroof to true if indeed any
player entity was on ground or on a roof.

Unfortunately, the way Leo did it was to make the two statements
mutually exclusive - an 'if'-'else if' instead of two separate
statements. So a single entity could not mark both any_onground and
any_onroof as true (and the majority of the time, you will be a single
entity).

Thus, the solution is to just drop that 'else'.

Fixes #855.

desktop_version/src/Input.cpp

@@ -2198,7 +2198,7 @@ void gameinput(void)
             {
                 any_onground = true;
             }
-            else if (obj.entities[ie].onroof > 0)
+            if (obj.entities[ie].onroof > 0)
             {
                 any_onroof = true;
             }