Deprecate Clip: remove rtree which is causing pip install issues

[lwasser]

i'm not sure we are even still using it so i'm going to test a local remove to see what happens.

this relates to #507 and should potentially fix it but we need to see how the docs, etc build as well

[lwasser]

failing because rtree is missing. and likely it can't install properly via pip as seen locally.
Because we are deprecating clip, we may be able to circle around this however because the clip tests are what is failing.

[codecov-io]

Codecov Report

Merging #509 into master will decrease coverage by 0.55%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
- Coverage      95%   94.44%   -0.56%     
==========================================
  Files          25       25              
  Lines        1660     1585      -75     
==========================================
- Hits         1577     1497      -80     
- Misses         83       88       +5

Impacted Files	Coverage Δ
earthpy/tests/test_clip.py	100% <100%> (ø)	⬆️
earthpy/clip.py	70.58% <50%> (-29.42%)	⬇️

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[lwasser]

ok. CI is failing here because codecov is simply not working. But this PR actually REMOVES an entire module and so cover coverage is not relevant for this PR. All testing is passing so i'll merge this even tho it looks "broken" it is not. ci is struggling today. as is my internet.