Remove RateLimit.rate

[s-t-e-v-e-n-k]

RateLimit.rate was first deprecated in v1.43.2, which was released 20
months ago. As that's a fairly generous deprecation period, remove it.

[codecov-commenter]

Codecov Report

Merging #1529 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1529      +/-   ##
==========================================
- Coverage   98.43%   98.43%   -0.01%     
==========================================
  Files          46       46              
  Lines        2174     2171       -3     
==========================================
- Hits         2140     2137       -3     
  Misses         34       34              

Impacted Files	Coverage Δ
github/RateLimit.py	100.00% <ø> (ø)

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