Remove the Shlex class now it is no longer needed

[ascopes]

GH-399 removed the need for the Shlex class anymore, and most of the complexity is able to be inlined/removed entirely now.

This PR totally removes the implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 78.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 87.76%. Comparing base (c66825b) to head (f92fb56).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...protobufmavenplugin/plugins/JvmPluginResolver.java	77.09%	10 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #400      +/-   ##
==========================================
- Coverage   89.05%   87.76%   -1.28%     
==========================================
  Files          34       33       -1     
  Lines        1159     1119      -40     
  Branches       81       74       -7     
==========================================
- Hits         1032      982      -50     
- Misses         93      103      +10     
  Partials       34       34              

Files with missing lines	Coverage Δ
...rotobufmavenplugin/protoc/CommandLineExecutor.java	77.28% <100.00%> (+0.68%)	⬆️
...protobufmavenplugin/plugins/JvmPluginResolver.java	84.51% <77.09%> (-6.17%)	⬇️