Remove redundant MyST labels that clash with doc targets

Remove (jax_intro)= from jax_intro.md and (autodiff)= from autodiff.md.
These labels duplicate the automatic :std:doc: targets created from the
filenames, causing myst.xref_ambiguous warnings that fail CI with -W.

Author: mmcky

lectures/autodiff.md

@@ -11,8 +11,6 @@ kernelspec:
   name: python3
 ---
 
-(autodiff)=
-
 # Adventures with Autodiff
 
 

lectures/jax_intro.md

@@ -11,8 +11,6 @@ kernelspec:
   name: python3
 ---
 
-(jax_intro)=
-
 # JAX
 
 This lecture provides a short introduction to [Google JAX](https://github.com/jax-ml/jax).