Cleanup code around JERRY_UNREACHABLEs (#2342)

`JERRY_UNREACHABLE`s often signal code structure that could be
improved: they can usually either be rewritten to `JERRY_ASSERT`s
or eliminated by restructuring loops, `if`s or `#if`s. Roughly,
the only valid occurences are in default cases of `switch`es. And
even they can often be merged into non-default cases.

Moreover, it is dangerous to write meaningful code after
`JERRY_UNREACHABLE` because it pretends as if there was a way to
recover from an impossible situation.

This patch rewrites/eliminates `JERRY_UNREACHABLE`s where possible
and removes misleading code from after them.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/ecma/operations/ecma-objects-general.c

@@ -169,18 +169,15 @@ ecma_op_general_object_delete (ecma_object_t *obj_p, /**< the object */
     /* b. */
     return ECMA_VALUE_TRUE;
   }
-  else if (is_throw)
+
+  /* 4. */
+  if (is_throw)
   {
-    /* 4. */
     return ecma_raise_type_error (ECMA_ERR_MSG ("Expected a configurable property."));
   }
-  else
-  {
-    /* 5. */
-    return ECMA_VALUE_FALSE;
-  }
 
-  JERRY_UNREACHABLE ();
+  /* 5. */
+  return ECMA_VALUE_FALSE;
 } /* ecma_op_general_object_delete */
 
 /**