Clean up some of the socket proxy code to simplify code

[atoulme]

fly by PR as I was rereading this code and found it could be tidied up a bit.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 44.91%. Comparing base (787a646) to head (689d824).

Files with missing lines	Patch %	Lines
tests/testutils/socket_proxy.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6256      +/-   ##
==========================================
+ Coverage   44.89%   44.91%   +0.02%     
==========================================
  Files         390      390              
  Lines       26994    26998       +4     
==========================================
+ Hits        12119    12127       +8     
+ Misses      14008    14004       -4     
  Partials      867      867              

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[jvoravong]

The failing tests look legitimate and related. Please look into them.

Error: receivers/mysql/bundled_test.go:35:2: declared and not used: dockerSocketProxy
Error: receivers/mysql/bundled_test.go:56:98: undefined: dockerSocket