Stop doing sparse stuff in this package

[willtebbutt]

AbstratGPs.jl does it, so Stheno.jl shouldn't also. It's unecessary maintenance.

[codecov-commenter]

Codecov Report

Merging #213 (c3e94fb) into master (577fce7) will decrease coverage by 0.22%.
The diff coverage is 100.00%.

❗ Current head c3e94fb differs from pull request most recent head 1d7e652. Consider uploading reports for the commit 1d7e652 to get more accurate results

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   86.65%   86.42%   -0.23%     
==========================================
  Files          17       17              
  Lines         442      442              
==========================================
- Hits          383      382       -1     
- Misses         59       60       +1     

Impacted Files	Coverage Δ
src/composite/cross.jl	97.36% <ø> (ø)
src/gaussian_process_probabilistic_programme.jl	100.00% <ø> (ø)
src/sparse_finite_gp.jl	93.33% <100.00%> (ø)
src/util/zygote_rules.jl	92.30% <0.00%> (-7.70%)	⬇️

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