chore: remove dead plasma-theme pkg, fix optdepends and dead cmake flag

- drop antergos-plasma-theme: superseded by antergos-layan-theme, the
  active look-and-feel (com.github.vinceliuice.Layan). Package had no
  dependents, was not referenced by any calamares config or netinstall
  group, and pulled plasma-desktop as a hard dep. Removed from
  packages.yaml, README, and deleted the package directory. The
  start-here-kde-antergos.svg it shipped is a self-contained copy inside
  antergos-layan-theme, so nothing breaks.
- antergos-next-desktop-settings pkgrel 1-4: add descriptions to
  optdepends (qt5ct/qt6ct) per Arch packaging guidelines (closes #2).

Calamares pkgrel 3.4.2-10:
- remove -DCMAKE_DISTRIBUTION_NAME: calamares ignores it; the 'for
  Antergos NeXT' about text comes from versionedName in the active
  branding component at runtime (closes #3).

packages/antergos-next-desktop-settings/PKGBUILD

@@ -2,7 +2,7 @@
 
 pkgname=antergos-next-desktop-settings
 pkgver=1
-pkgrel=3
+pkgrel=4
 pkgdesc="Antergos NeXT desktop settings, theme, and color scheme"
 arch=('any')
 url="https://github.com/Antergos-NeXT/antergos-packages"
@@ -14,7 +14,7 @@ depends=(
   'noto-fonts'
   'ttf-roboto'
 )
-optdepends=('qt5ct' 'qt6ct')
+optdepends=('qt5ct: Qt5 configuration tool' 'qt6ct: Qt6 configuration tool')
 source=(
   "antergos-next-gtk3.css"
   "antergos-next-gtk4.css"