Use completing-read-default in tmm-prompt

Use completing-read-default in tmm-prompt

tmm uses completing-read, but customizes its behavior so much
that any alternative completing-read-function will almost
certainly break it.  For example, both ido-ubiquitous and ivy have
special code to deactivate themselves for tmm.
* lisp/tmm.el (tmm-prompt): Use completing-read-default instead of
completing-read.  (Bug#27193)

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