Remove Migration Admonitions

[mmcky]

There are a bunch of migration admonitions that we can now delete and remove

This is encoded as

```{admonition} Migrated lecture
:class: warning

This lecture has moved from our [Intermediate Quantitative Economics with Python](https://python.quantecon.org/intro.html) lecture series and is now a part of [A First Course in Quantitative Economics](https://intro.quantecon.org/intro.html).
```

in the md files.

lectures/ar1_processes.md::class: warning
lectures/complex_and_trig.md::class: warning
lectures/geom_series.md::class: warning
lectures/lp_intro.md::class: warning
lectures/schelling.md::class: warning
lectures/short_path.md::class: warning

[HumphreyYang]

I think this is closed by @longye-tian with #466.