Auto merge of #116506 - Wilfred:remove_tmp_var, r=workingjubilee

Remove unnecessary tmp variable in default_read_exact

This `tmp` variable has existed since the original implementation (added in ff81920f03866674080ac63b565cc9d30f80c450), but it's not necessary (maybe non-lexical lifetimes helped?).

It's common to read std source code to understand how things actually work, and this tripped me up on my first read.

Author: bors