fix: drop stale ax= kwarg from ArmPlot's robot.plot() call (#590)

BaseRobot.plot() no longer accepts an ax= parameter -- only fig= is a
real, documented parameter; ax isn't referenced anywhere in its body or
docstring, and flows straight through into env.add(), whose signature
(PyPlot.add()) has never accepted ax either. Found via bdsim's ARMPLOT
block, which asserts self.ax is not None and then passes ax=self.ax
into robot.plot(), raising TypeError: PyPlot.add() got an unexpected
keyword argument 'ax'.

self.ax isn't used anywhere else in arm.py -- dropping it from this one
call site is sufficient, robot.plot() manages its own axes internally
given fig=.

Found while investigating a bdsim example (RVC3-python's RRMC.py) that
used to work in the book (2022) -- this was the second of two stacked
bugs blocking it end to end, the first being bdsim's own PROD(matrix=)
removal (unrelated, fixed on the notebook side).

Author: petercorke

src/roboticstoolbox/blocks/arm.py

@@ -416,7 +416,6 @@ def start(self, simstate):
             self.q0,
             backend=self.backend,
             fig=self.fig,
-            ax=self.ax,
             block=False,
         )