PubChem: simplify `load_pubchem_json`

[tkemmer]

Fixes: #88

• 1.9.4

  julia> @time load_pubchem_json("test/data/aspirin_pug.json")
  0.770264 seconds (388.79 k allocations: 26.181 MiB, 99.46% compilation time)
  System with 21 atoms

• 1.10.4

  julia> @time load_pubchem_json("test/data/aspirin_pug.json")
  0.640385 seconds (323.58 k allocations: 22.202 MiB, 99.65% compilation time)
  System with 21 atoms

• 1.11.0-rc2

  julia> @time load_pubchem_json("test/data/aspirin_pug.json")
  2.163406 seconds (1.21 M allocations: 58.932 MiB, 1.82% gc time, 99.89% compilation time)
  System with 21 atoms

• 1.12.0-DEV.1096 (03451ff827a)

  julia> @time load_pubchem_json("test/data/aspirin_pug.json")
  1.964002 seconds (1.19 M allocations: 60.537 MiB, 0.42% gc time, 99.89% compilation time: 3% of which was recompilation)
  System with 21 atoms