remove xgraph from ngspice distribution.

ngspice supports gnuplot instead.
30 year old legacy code, not maintained.
Does not compile on MS Windows,
compiles on Cygwin, but xgraph.exe crahes immediately.
Lots of compiler warnings.

configure.ac

@@ -185,11 +185,6 @@ AC_ARG_ENABLE([cluster],
 AC_ARG_ENABLE([help],
     [AS_HELP_STRING([--enable-help], [Force building nghelp (deprecated)])])
 
-# --enable-xgraph: Compile the xgraph plotting program. Default is "no".
-# Xgraph is a plotting package for X11 once very popular.
-AC_ARG_ENABLE([xgraph],
-    [AS_HELP_STRING([--enable-xgraph], [Enable xgraph compilation.])])
-
 # --with-fftw3: Use fftw3 for Fourier transforms.  Default is "yes".
 AC_ARG_WITH([fftw3],
     [AS_HELP_STRING([--with-fftw3[=yes/no]], [Use fftw3 for Fourier transforms. Default=yes.])])
@@ -912,18 +907,6 @@ if test "x$enable_smltmsdebug" = xyes; then
     AC_DEFINE([D_DBG_SMALLTIMES], [], [Define if we want debug distortion analysis (SMALLTIMES)])
     AC_MSG_RESULT([WARNING: Distortion analysis debug *D_DBG_SMALLTIMES* is enabled])
 fi
-if test "x$enable_xgraph" = xyes; then
-    AC_MSG_RESULT([Xgraph compilation enabled.])
-    AC_CONFIG_SUBDIRS([xgraph])
-    XGRAPHDIR="xgraph"
-    NOTXGRAPH=""
-else
-    XGRAPHDIR=""
-    NOTXGRAPH="xgraph"
-fi
-
-AC_SUBST([XGRAPHDIR])
-AC_SUBST([NOTXGRAPH])
 
 AC_PROG_YACC
 AC_PATH_PROG([BISON], [bison])