[EH] Remove !rethrown check when destroying exception

In Emscripten EH, #25988 changed where we increment refcount to fix a
refcounting inconsistency between Emscripten vs. Wasm EH (#17115).
Before #25988, it was incremented in `__cxa_begin_catch`, but after
 #25988, it was in `__cxa_throw` and `__cxa_rethrow`, more inline with
Wasm EH.

`__cxa_end_catch` calls `__cxa_decrement_exception_refcount`, which
decrements the refcount and destroys/frees the exception. But when an
exception is rethrown, we should not destroy it in `__cxa_end_catch`.
The below illutrates how a rethrown exception is (correctly) not
destroyed in `__cxa_end_catch`:
```cpp
// Before #25988 (refcount incremented in __cxa_begin_catch):
try {
  throw MyEx(); // __cxa_throw:       refcount = 0, rethrown = false
} catch (...) { // __cxa_begin_catch: refcount = 1 (0 -> 1), rethrown = false
  throw;        // __cxa_rethrow:     refcount = 1 (unchanged), rethrown = true
}               // __cxa_end_catch:   refcount = 0 (1 -> 0); not destroyed/freed because rethrown == true

// After #25988 (refcount incremented in __cxa_throw and __cxa_rethrow):
try {
  throw MyEx(); // __cxa_throw:       refcount = 1 (0 -> 1)
} catch (...) { // __cxa_begin_catch: refcount = 1 (unchanged)
  throw;        // __cxa_rethrow:     refcount = 2 (1 -> 2)
}               // __cxa_end_catch:   refcount = 1 (2 -> 1); not destroyed/freed because refcount > 0
```

So after #25988 we were supposed to solely rely on the refcount to
determine whether to destroy an exception. But the
`!exception_header->rethrown` check was incorrectly still left in
`__cxa_decrement_exception_refcount`. This didn't affect C++-only
programs, but this prevented an exception from being destroyed when an
exception was rethrown to JS and `decrementExceptionRefcount` was called
from there, as in the test added here.

This removes the incorrect `rethrown` flag check. After this and #26518,
no code uses the `rethrown` field, so this also removes it and its
related methods.

This is a part of fixes for #27705.