unreachable code for REMU in rv_alu.h

[ilyakurdyukov]

https://github.com/mortbopet/Ripes/blob/878087332afa3558dc8ca657f80a16ecdcf82818/src/processors/RISC-V/rv_alu.h#L105

This is literally unreachable code that was copied from DIV to REMU. GCC seems to be blind to notice this, but the EDG frontend sees this.