Don't include PowerShell completions in tgz

The PowerShell completion script was outdated,
and removed from this repository in
65fdbf0210.

A more up to date implementation can be found
here; https://github.com/samneirinck/posh-docker

Removing this script from the tgz

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/make/tgz

@@ -58,7 +58,7 @@ for d in "$CROSS/"*/*; do
 	copy_binaries $TAR_PATH
 
 	# add completions
-	for s in bash fish powershell zsh; do
+	for s in bash fish zsh; do
 		mkdir -p $TAR_PATH/completion/$s
 		cp -L contrib/completion/$s/*docker* $TAR_PATH/completion/$s/
 	done