remove --loader arg and config options

[realFlowControl]

This PR removes the --loader command line argument as well as the testSuiteLoaderClass and testSuiteLoaderFile configuration options as stated in #4524

[codecov]

Codecov Report

Merging #4531 (92fbf4f) into master (d86b1ee) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #4531      +/-   ##
============================================
+ Coverage     84.31%   84.35%   +0.03%     
+ Complexity     4625     4613      -12     
============================================
  Files           275      275              
  Lines         11964    11932      -32     
============================================
- Hits          10088    10065      -23     
+ Misses         1876     1867       -9     

Impacted Files	Coverage Δ	Complexity Δ
src/TextUI/CliArguments/Builder.php	80.18% <ø> (+0.27%)	121.00 <0.00> (-1.00)	⬆️
src/TextUI/CliArguments/Mapper.php	79.51% <ø> (+0.35%)	82.00 <0.00> (-1.00)	⬆️
src/TextUI/XmlConfiguration/Loader.php	97.59% <ø> (-0.02%)	134.00 <0.00> (-1.00)
src/Runner/BaseTestRunner.php	86.20% <100.00%> (+5.56%)	9.00 <0.00> (-1.00)	⬆️
src/TextUI/CliArguments/Configuration.php	68.58% <100.00%> (+0.28%)	269.00 <1.00> (-3.00)	⬆️
src/TextUI/Command.php	74.59% <100.00%> (-0.17%)	118.00 <0.00> (-3.00)
src/TextUI/TestRunner.php	66.23% <100.00%> (-0.44%)	226.00 <2.00> (-2.00)
src/TextUI/XmlConfiguration/PHPUnit/PHPUnit.php	96.51% <100.00%> (-0.05%)	62.00 <1.00> (ø)

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[sebastianbergmann]

I will merge this once master is PHPUnit 10 (after December 4, 2020).

[sebastianbergmann]

Merged manually, thanks!