Fix debian-minimal image building

'locales' package gets installed before '12-debian-locale-gen' is executed
and generates effectively empty /etc/locales.gen in debian, which makes
dpkg-reconfigure call to locales ignore the values set by
debconf-set-selections.
* Remove /etc/locale.gen generated by 'locales' installation to ensure
  proper locales generation on debian images
* Remove 'locales-all' package installation from debian element since
  it's not needed anymore to build the image and cosnumes additional
  ~120MB of space
* Remove unused 'package-installs' dependency from debian-minimal
  element

Change-Id: Ic39ba2b5ceb5018efb75742547b2babf80827e56
Closes-Bug: #1452400

elements/debian-minimal/element-deps

@@ -1,2 +1 @@
 debootstrap
-package-installs

elements/debian/package-installs.yaml

@@ -10,4 +10,3 @@ net-tools:
 cloud-init:
 cloud-utils:
 cloud-initramfs-growroot:
-locales-all:

elements/debootstrap/install.d/12-debian-locale-gen

@@ -21,6 +21,8 @@ fi
 set -eu
 set -o pipefail
 
+[ -f /etc/locale.gen ] && rm /etc/locale.gen
+
 debconf-set-selections - <<EOF
 locales locales/locales_to_be_generated multiselect en_US.UTF-8 UTF-8
 # Choices: None, C.UTF-8, en_US.UTF-8