Remove math module from count

[nikolai-b]

Not required after rails/arel#449

There is a test showing it isn't needed here:
https://github.com/rails/rails/blob/master/activerecord/test/cases/arel/nodes/count_test.rb#L36-L42

Copied over from rails/arel#525

[rails-bot]

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[sinsys1981]

test