Revert "C backend: Support struct bitcasts."

This reverts commit d7782f5.
Not needed after all; also, I forgot I didn't want to support this.

src/backend/c.nt

@@ -656,19 +656,12 @@ class CBackendFunction : BackendFunction {
                     }
                 }
                 (:bitCast, int from, BackendType to): {
-                    auto ssaInstr = this.instrs[from];
-                    bool fake = ssaInstr.mode == SSAMode.held && ssaInstr.heldValue.fake;
-                    if (fake) ret = Value.none;
-                    else if (to.instanceOf(BackendStructType)) {
-                        flushRegs; // put 'from' into a variable
-                        string toFmt = typeFmt(mod, to);
-                        string newVal = "*($toFmt*) &reg$from";
-                        ret = Value(toFmt, newVal, exactType=true);
-                    } else {
-                        auto fromExt = regExt(from);
+                    auto from = regExt(from);
+                    if (from.fake) ret = Value.none;
+                    else {
                         string typestr = typeFmt(mod, to);
 
-                        ret = Value(typestr, fromExt.value, exactType=false);
+                        ret = Value(typestr, from.value, exactType=false);
                     }
                 }
                 (:zeroExtend, int value, BackendType to): {