fix/345: clean up redundant code

[novahe]

isVowel应该是单数合适一些
continue 可以减少if分支判断

[codecov-commenter]

Codecov Report

Merging #127 (2450cf4) into master (2482928) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
- Coverage   71.82%   71.81%   -0.01%     
==========================================
  Files         673      673              
  Lines       11953    11950       -3     
==========================================
- Hits         8585     8582       -3     
  Misses       3056     3056              
  Partials      312      312              

Impacted Files	Coverage Δ
...els-of-a-String/345. Reverse Vowels of a String.go	100.00% <100.00%> (ø)

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