Whether the start and end times are displayed when entering time is determined by the Units on the pay code and/or charge type for the work that is selected. If, for example you have a pay code and change type with units of 'days' then when you enter time against work that is linked to that pay code / charge type the start and end time are removed and 'days' is shown as the prompt. It is only when the units for the pay code / charge type is 'Hours' that the start and end times are shown.
So to allow the user to just enter hours without start and end times, you could change the units from the default 'Hours' to, say, 'total hours' or even just 'hours' (it is case sensitive).
Note that this will not give you the 7 day entry form that you get when you add a time entry with 'Record Hours Only' ticked - you still need to add a time entry per day. But it will allow you to just add the total hours for the week on a Friday since the maximum 24 hour restriction on regular Hours entries will not apply.
You do have to be careful when using this approach for regular employees who receive leave based on the number of hours they work. For FlexiTime to recognise time as hours for which leave is accrued you need to have the units set to 'Hours' or 'hours'.