Remove three duplicate bibliography entries so the .bib parses at all (#204)

pybtex refuses to parse a bibliography containing a repeated key, and this file
has carried three of them — GilboaSchmeidler:1989, Whittle_1981 and Whittle_1990
each appear twice, once indented in the main body and once unindented near the
end. The two copies of each are byte-identical apart from leading whitespace.

The effect is total rather than local: `parse_file` raises
`BibliographyDataError: repeated bibliography entry` and no bibliography is
built, so *every* {cite} in the edition fails to resolve. That is what the
"could not find bibtex key" warnings on #202 actually are — the keys are present
in the file, but nothing ever parsed it. The 70 entries backfilled in #203 were
correct and could never have taken effect while this stood.

Predates #203: the same three duplicates and the same parse error are present at
693aa63, before that PR.

Verified: pybtex now parses 480 entries, and all 61 keys the #202 build reported
missing resolve, as do the three formerly-duplicated ones.

Removes the later copy of each; the earlier copy in the main body is kept.

Author: mmcky

lectures/_static/quant-econ.bib

@@ -4104,43 +4104,6 @@ @article{Campbell1987
   pages   = {1249--1273},
   year    = {1987}
 }
-
-@article{GilboaSchmeidler:1989,
- 	author = {Gilboa, Itzhak and Schmeidler, David},
- 	date-added = {2020-08-10 09:11:02 -0500},
- 	date-modified = {2020-08-10 09:11:02 -0500},
- 	journal = {Journal of Mathematical Economics},
- 	keywords = {climate,modeling},
- 	mendeley-groups = {nsfbib},
- 	month = {apr},
- 	number = {2},
- 	pages = {141--153},
- 	title = {{Maxmin Expected Utility with Non-Unique Prior}},
- 	volume = {18},
- 	year = {1989}
-}
-
-@article{Whittle_1981,
-  author = {Peter Whittle},
-  year = {1981},
-  title = {Risk-Sensitive Linear/Quadratic/Gaussian Control},
-  journal = {Advances in Applied Probability},
-  volume = {13},
-  number = {4},
-  pages = {764-777}
-}
-
-@book{Whittle_1990,
-  author = {Peter Whittle},
-  title = {Risk-Sensitive Optimal Control},
-  year = {1990},
-  publisher = {Wiley},
-  address = {New York}
-}
-
-
-
-
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