build-goo: Remove vestigial remnants of old GDM integration code.

commit 04102125 removed a feature, only several versions of
GDM relied on for doing the splash to login screen transition.
but it missed a few spots in the build goo and documentation. So
this commit does another pass to fully clean up any lingering
references to obsolete GDM transition code.

INSTALL

@@ -23,7 +23,6 @@ So the basic idea is you need to:
          --with-background-start-color-stop=0x0073B3           \
          --with-background-end-color-stop=0x00457E             \
          --with-background-color=0x3391cd                      \
-         --enable-gdm-transition                               \
          --with-system-root-install                            \
          --with-rhgb-compat-link
 

configure.ac

@@ -110,8 +110,6 @@ if test x$enable_tracing = xyes; then
   AC_DEFINE(PLY_ENABLE_TRACING, 1, [Build in verbose debug tracing spew])
 fi
 
-AC_ARG_ENABLE(gdm-transition, AS_HELP_STRING([--enable-gdm-transition],[enable smooth transition to gdm]),enable_gdm_transition=$enableval,enable_gdm_transition=no)
-
 AC_ARG_ENABLE(upstart-monitoring, AS_HELP_STRING([--enable-upstart-monitoring],[listen for messages on the Upstart D-Bus interface]),enable_upstart_monitoring=$enableval,enable_upstart_monitoring=no)
 if test x$enable_upstart_monitoring = xyes; then
   PKG_CHECK_MODULES(DBUS, [dbus-1])

scripts/plymouth.spec

@@ -222,7 +222,6 @@ sed -i -e 's/spinner/charge/g' src/plymouthd.defaults
            --with-background-start-color-stop=0x0073B3           \
            --with-background-end-color-stop=0x00457E             \
            --with-background-color=0x3391cd                      \
-           --disable-gdm-transition                              \
            --enable-systemd-integration                          \
            --without-system-root-install                         \
            --without-log-viewer					 \