Remove memset of the UE TX time domain samples array after trx_write in
RU_write()
This memset is unnecessary and can break openair0 libraries. Some
functionalities, e.g. usrp_lib write_thread expect the samples to be
present in the array after trx_write returns.
The array is always set before being sent either in idft or using memset
so there is no risk of sending the same data twice.