src,test: Fix readability-else-after-return warnings

[stotko]

else statements after a return in the preceding if clause seem intuitive at first glance since they follow natural language. However, the else statement is superfluous from a technical point of view. Follow clang-tidy recommendations and drop these statements.

[codecov]

Codecov Report

Merging #133 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #133   +/-   ##
=======================================
  Coverage   97.63%   97.63%           
=======================================
  Files          28       28           
  Lines        1694     1694           
=======================================
  Hits         1654     1654           
  Misses         40       40           

Impacted Files	Coverage Δ
src/stdgpu/impl/deque_detail.cuh	93.56% <100.00%> (ø)
src/stdgpu/impl/unordered_base_detail.cuh	96.49% <100.00%> (ø)
src/stdgpu/impl/vector_detail.cuh	93.87% <100.00%> (ø)

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