Remove unnecessary illegal-instruction checks

All of these are cases where other instructions overlay the reserved
encodings (e.g. C.EBREAK is C.JALR with rs1=x0), so the checks are
redundant.

riscv/insns/c_add.h

@@ -1,3 +1,2 @@
 require_extension(EXT_ZCA);
-require(insn.rvc_rs2() != 0);
 WRITE_RD(sext_xlen(RVC_RS1 + RVC_RS2));

riscv/insns/c_jalr.h

@@ -1,5 +1,4 @@
 require_extension(EXT_ZCA);
-require(insn.rvc_rs1() != 0);
 reg_t tmp = npc;
 set_pc(RVC_RS1 & ~reg_t(1));
 WRITE_REG(X_RA, tmp);

riscv/insns/c_mv.h

@@ -1,3 +1,2 @@
 require_extension(EXT_ZCA);
-require(insn.rvc_rs2() != 0);
 WRITE_RD(RVC_RS2);