Remove the `gadm` function

[eliascarv]

The gadm function has been moved to the GeoArtifacts.jl package.

[codecov[bot]]

Codecov Report

Merging #16 (330ef93) into master (01e47f2) will decrease coverage by 0.78%. Report is 1 commits behind head on master. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   92.61%   91.83%   -0.78%     
==========================================
  Files           9        8       -1     
  Lines         203      196       -7     
==========================================
- Hits          188      180       -8     
- Misses         15       16       +1     

Files	Coverage Δ
src/GeoIO.jl	100.00% <ø> (ø)

... and 1 file with indirect coverage changes