DblPend, SglPend: `A:startOrigin` assumptions++

[balacij]

@smith's recent comment on #4088 (https://github.com/JacquesCarette/Drasil/pull/4088#discussion_r2116223757) had me looking into the assumptions into A:startOrigin assumption a tad bit, where I realized that:

1. A:startOrigin doesn't appear to be used.
2. It also seems to be superseded by A:firstPend, which includes the content of A:startOrigin with the "first pendulum" text.
3. However, A:firstPend is also not referenced anywhere in the SRS!

[smiths]

@balacij thank you for the new information. I think the fact that the assumption is never invoked is evidence that it isn't really necessary. The assumption is an incomplete version of what we already say when we define the angle $\theta_1$. We are saying in words what we say better in the figure showing the double pendulum. The same applies for A:secondPend and $\theta_2$.

I think the cleanest fix is to remove the two assumptions we are discussing.

[jackwyand]

I was going to work on this however I just wanted to confirm that the result of this conversation was to just remove the startOrigin, firstPend, and secondPend assumptions altogether?

[smiths]

Yes, you can remove them @jackwyand.