[rtl] Remove incorrect LSU assertion
[rtl] Remove incorrect LSU assertion

- Protocol-wise data_err_i is notionally X when !data_rvalid_i
- In addition, the design does not appear to rely on the asserted
  behaviour
- Removing as it is firing in chip-level OT simulations

Signed-off-by: Tom Roberts <f3b1a53ee1cbeb35a47033dea932030093853292@lowrisc.org>

lowRISC/ibex,AmbiML/ibex,lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex