RISC-V: Remove XFAIL of ssa-dom-cse-2.c

Confirm RISC-V is able to CSE this case no matter whether we enable RVV or not.

Remove XFAIL,  to fix:
XPASS: gcc.dg/tree-ssa/ssa-dom-cse-2.c scan-tree-dump optimized "return 28;"

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/ssa-dom-cse-2.c: Remove riscv.