rubocop: don’t allow Perl regex backrefs.

Even if shell/perl users recognise these variables the explicit versions
are significantly more readable for everyone.

Library/.rubocop.yml

@@ -176,10 +176,6 @@ Style/PercentLiteralDelimiters:
     '%W': '[]'
     '%x': '()'
 
-# we prefer Perl-style regex back references
-Style/PerlBackrefs:
-  Enabled: false
-
 Style/RaiseArgs:
   EnforcedStyle: exploded