Remove class alloc check

This checks that the value returned from the function registered with
rb_define_alloc_func is of the correct class. When this was first
introduced in 1fe40b7cc5 (by Matz on 2001-10-03), allocation was done
via user-defined Object#allocate, so it made sense to have a runtime
check in release builds. Now that it's defined via rb_define_alloc_func
in the C extension API, I don't think it's necessary.

The check is surprisingly expensive. Removing it makes Object.new about
10% faster. It allows the C compiler to optimize the call to the
function pointer as a tail-call. Removing this also allows ZJIT/YJIT to
call the function directly (ZJIT already does this by having a list of
known safe allocation functions).

There's no way for users to ever have seen this check, other than by
writing a misbehaving C extension, which returns objects with the wrong
class.

[Feature #21966]

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