completion.el: 1) has unnecessary macro code, 2) has no way to turn it off (?)

[Drew Adams]

1) IIUC, all of the calls to macro `cmpl-statistics-block' should be removed
from this library.

The macro does nothing, and most of the args passed to it (which are
ignored) are undefined anyway.

The comment introducing the macro says that the functionality is defined in
library `completion-stats.el', which no longer exists, AFAICT.

1) IIUC, once you call `dynamic-completion-mode' to turn on completion,
there is no way to turn it off. This should be made into a proper minor
mode.