Revert "wmflib: add to_milliseconds() / to_seconds()"

This reverts commit 191dda5d7f593ec36df31f1b4d8fc33a950f4ca5.

Change-Id: Ifd400578bac011e138d472035207d8264a586e58
Merge "Revert "wmflib: add to_milliseconds() / to_seconds()"" into production