Remove Contexts for the sack of simplicity

[sunxd3]

The three evaluation functions for BUGSModel can be distinguished without Contexts.

This PR removes them, as needs arise, we can add them back.

[coveralls]

Pull Request Test Coverage Report for Build 11518416809

Details

• 9 of 14 (64.29%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 77.891%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/model.jl	5	10	50.0%

Totals
Change from base Build 11517551771:	0.07%
Covered Lines:	1381
Relevant Lines:	1773

---

💛 - Coveralls

[github-actions]

BridgeStan not found at location specified by $BRIDGESTAN environment variable, downloading version 2.5.0 to /home/runner/.bridgestan/bridgestan-2.5.0
Done!
Model dogs produces error: ErrorException("log_density() failed with exception: Exception: bernoulli_lpmf: Probability parameter is inf, but must be in the interval [0, 1] (in '/home/runner/work/JuliaBUGS.jl/JuliaBUGS.jl/benchmark/stan/bugs_examples/vol1/dogs/dogs.stan', line 37, column 6 to line 38, column 62)\n")

Model	Parameter Count	Data Count	Stan Density Time (µs)	Stan Density Gradient Time (µs)	JuliaBUGS Density Time with Graph Walk (µs)	JuliaBUGS Density Gradient Time with ReverseDiff.jl(compiled tape) (µs)
rats	65	150	5.6024	8.67933	286.44	87.021
pumps	12	10	1.00045	1.37118	31.057	6.587
dogs	2	720	NA	NA	736.911	149.305
seeds	26	21	2.44867	3.25044	73.856	20.919
surgical_realistic	14	12	1.24604	1.69092	40.164	8.439
magnesium	108	96	10.6145	12.233	371.562	91.429
salm	22	18	2.35108	2.98111	53.529	12.393
equiv	15	20	2.47708	3.35625	48.79	15.097
dyes	9	30	0.987833	1.31195	30.596	13.094
stacks	6	21	1.14532	1.67847	97.19	15.298
epil	303	236	32.891	39.323	863.336	267.009
blockers	47	44	3.31167	3.75843	164.604	28.242
oxford	244	240	16.19	18.755	1055.83	174.513
lsat	1006	5000	174.403	214.497	8729.44	1228.34
bones	33	422	72.8595	90.137	2478.67	199.959
mice	20	65	7.3085	9.324	74.217	37.82
kidney	64	58	10.8	16.321	191.344	71.5925
leuk	18	714	20.468	26.789	870.856	203.637
leukfr	40	714	23.974	32.29	911.641	265.561