Remove "Show macro list" keybinding and feature

This feature looks like a poorly implemented subset of "Complete word",
but lacks some important features like prefix filtering.  It is also
misnamed as it shows not only macros, but also variables and enums.

It also only shows `editor_prefs.autocompletion_max_entries`, but does
so from *each* file, not as a whole.

So drop it altogether, as this feature doesn't really look useful and
the current implementation seem to suffer of too many shortcomings for
it to realistically be actually used.

See [discussion in #356](/geany/geany/pull/356#issuecomment-61737645)
@eht16 @ntrel @codebrainz any opinion on why it exists and removing it?