fix(completion): removed erroneous completion var

The definition of the variable is using invalid bash syntax and it is unused anyways.

$ _binary_arch=${DEVTOOLS_VALID_ARCHES[*]:0:-1}
bash: -1: substring expression < 0

Fixes #222 (closed)

Fixes: f961e2e9 ("completion: implemented structured declarative bash completions") Signed-off-by: Christian Heusel christian@heusel.eu

Closes #222 (closed)