Remove Performance/Detect rubocop rule

After upgrading the rubocop-performance gem, we're asked to change this
code:

milestones.select { |milestone| milestone.image.present? }.last

To:

milestones.reverse.find { |milestone| milestone.image.present? }

IMHO the original code is easier to read, so the performance gain isn't
worth it.

.rubocop.yml

@@ -186,9 +186,6 @@ Lint/UselessAssignment:
 Performance/CompareWithBlock:
   Enabled: true
 
-Performance/Detect:
-  Enabled: true
-
 Performance/DoubleStartEndWith:
   Enabled: true