Remove dependency on Boost::regex

[jslee02]

The implementation bug of GCC is no longer issue since DART requires GCC >= 5.3.1.

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[codecov]

Codecov Report

Merging #1412 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1412      +/-   ##
==========================================
+ Coverage   57.31%   57.32%   +0.01%     
==========================================
  Files         367      367              
  Lines       27521    27521              
==========================================
+ Hits        15774    15777       +3     
+ Misses      11747    11744       -3

Impacted Files	Coverage Δ
dart/common/Uri.cpp	68.82% <ø> (ø)	⬆️
dart/utils/XmlHelpers.cpp	60.84% <ø> (ø)	⬆️
dart/dynamics/Skeleton.cpp	66.22% <0%> (+0.16%)	⬆️