Remove some ancient workarounds for very old Xdebug versions

[dvdoug]

Part of the work on #380, as discussed in #745

[codecov]

Codecov Report

Merging #746 into master will increase coverage by 0.59%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #746      +/-   ##
============================================
+ Coverage     81.96%   82.56%   +0.59%     
+ Complexity      843      835       -8     
============================================
  Files            36       36              
  Lines          2484     2466      -18     
============================================
  Hits           2036     2036              
+ Misses          448      430      -18     

Impacted Files	Coverage Δ	Complexity Δ
src/Driver/Xdebug.php	0.00% <ø> (ø)	7.00 <0.00> (-8.00)

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