configure: remove slirp submodule support that doesn't exist yet

The slirp code is not yet split off into a separate repository, so
configuring QEMU to use slirp as a submodule is premature. This
causes the non-existant "slirp" to be requested from git when syncing
submodules. This in turn appears to be cause of non-deterministic
failures some developers are seeing with QEMU's submodule sync process.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190313173157.30504-1-berrange@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>

configure

@@ -1109,8 +1109,6 @@ for opt do
   ;;
   --disable-slirp) slirp="no"
   ;;
-  --enable-slirp=git) slirp="git"
-  ;;
   --enable-slirp=system) slirp="system"
   ;;
   --disable-vde) vde="no"
@@ -5780,8 +5778,6 @@ case "$slirp" in
   "" | yes)
     if $pkg_config slirp; then
       slirp=system
-    elif test -e "${source_path}/.git" && test $git_update = 'yes' ; then
-      slirp=git
     elif test -e "${source_path}/slirp/Makefile" ; then
       slirp=internal
     elif test -z "$slirp" ; then
@@ -5799,10 +5795,7 @@ case "$slirp" in
 esac
 
 case "$slirp" in
-  git | internal)
-    if test "$slirp" = git; then
-      git_submodules="${git_submodules} slirp"
-    fi
+  internal)
     mkdir -p slirp
     slirp_cflags="-I\$(SRC_PATH)/slirp/src -I\$(BUILD_DIR)/slirp/src"
     slirp_libs="-L\$(BUILD_DIR)/slirp -lslirp"
@@ -6464,7 +6457,7 @@ if test "$slirp" != "no"; then
   echo "SLIRP_CFLAGS=$slirp_cflags" >> $config_host_mak
   echo "SLIRP_LIBS=$slirp_libs" >> $config_host_mak
 fi
-if [ "$slirp" = "git" -o "$slirp" = "internal" ]; then
+if [ "$slirp" = "internal" ]; then
     echo "config-host.h: subdir-slirp" >> $config_host_mak
 fi
 if test "$vde" = "yes" ; then