Clean up warnings

[boydm]

Description

Clean up compile warnings under Elixir 1.11. This does not change any functionality.

Motivation and Context

Types of changes

• [ x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

Checklist

• [ ] Check other PRs and make sure that the changes are not done yet.
• [ ] The PR title is no longer than 64 characters.

[codecov[bot]]

Codecov Report

Merging #202 into master will increase coverage by 0.01%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
+ Coverage   86.65%   86.66%   +0.01%     
==========================================
  Files          87       87              
  Lines        2338     2340       +2     
==========================================
+ Hits         2026     2028       +2     
  Misses        312      312              

Impacted Files	Coverage Δ
lib/scenic.ex	14.28% <ø> (ø)
lib/scenic/cache/base.ex	77.95% <0.00%> (ø)
lib/scenic/cache/support/supervisor.ex	100.00% <100.00%> (ø)
lib/scenic/view_port/supervisor_top.ex	25.00% <100.00%> (+25.00%)	:arrow_up:

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