Revert "Allow Hessian functors to return Hessian as compressed sparse matrix"

[SteveBronder]

Reverts #3055

@andrjohns thinking about this the sparse matrix is only going to save more memory if nnz < (n * (n - 1) - 1) / 2. But we have a lower triangular which takes up n * (n-1) / 2 entries. So idt this is actually saving any memory

[WardBrian]

Especially with the release next week it seems safer to revert if there are remaining questions

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
arma/arma.stan	0.23	0.21	1.13	11.69% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.01	0.01	0.95	-4.85% slower
gp_regr/gen_gp_data.stan	0.03	0.02	1.47	32.08% faster
gp_regr/gp_regr.stan	0.12	0.11	1.05	4.92% faster
sir/sir.stan	83.2	83.49	1.0	-0.34% slower
irt_2pl/irt_2pl.stan	4.38	4.26	1.03	2.78% faster
eight_schools/eight_schools.stan	0.08	0.05	1.54	35.09% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.27	0.28	0.98	-2.52% slower
pkpd/one_comp_mm_elim_abs.stan	19.39	19.56	0.99	-0.89% slower
garch/garch.stan	0.5	0.68	0.74	-35.95% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.02	2.95	1.02	2.34% faster
arK/arK.stan	1.75	1.67	1.05	4.52% faster
gp_pois_regr/gp_pois_regr.stan	2.7	2.62	1.03	3.19% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	9.84	9.47	1.04	3.82% faster
performance.compilation	222.33	209.19	1.06	5.91% faster
Mean result: 1.072324490650238

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Jenkins Console Log
Blue Ocean
Commit hash: 8229477b6234d7c8fc557a960b090f5699c54683

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G++:
g++ (Ubuntu 9.4.0-1ubuntu1~20.04) 9.4.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Clang:
clang version 10.0.0-4ubuntu1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[andrjohns]

only going to save more memory if nnz < (n * (n - 1) - 1) / 2.

Ah good to know, good catch!