Make drop_na.SpatRaster internal

[dieghernan]

No description provided.

[dieghernan]

See #1

[codecov]

Codecov Report

Merging #13 (4d1324e) into main (b49a1c6) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #13   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files          22       22           
  Lines         992      992           
=======================================
  Hits          988      988           
  Misses          4        4           

Impacted Files	Coverage Δ
R/replace_na.Spat.R	100.00% <ø> (ø)
R/drop_na.Spat.R	100.00% <100.00%> (ø)

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