Fix memory unsafety in Reader::push by simply removing it

[tbu-]

The memory unsafety was caused by the fact that it passed an uninitialized
buffer to a different function (Reader::read) which might not give the
(unsafe) guarantee that it doesn't touch the uninitalized bytes.

If one removes this unsafety, the function is no better than creating an
initialized vector and passing it to the Reader::read function, so it was
removed completely.

Fixes #21896.

[rust-highfive]

r? @nikomatsakis

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[tbu-]

See also #21076.

[tbu-]

r? @alexcrichton

[alexcrichton]

For now I think we'd prefer to avoid breaking changes to old_io wherever possible. This will naturally get phased out over time and the new std::io implementation is hopefully [soon to land]#21835) (which does not have this problem).

If you'd like to submit a fix for this though (using the black_box trick), then feel free to reopen!