Remove unused ENABLE_GC_FRIENDLY_DEFAULT and its build option

ENABLE_GC_FRIENDLY_DEFAULT was supposed to set the default for the gc friendliness
while still allowing to force enable it at runtime with G_DEBUG=gc-friendly.

With commit 943a18b564 (6 years ago) things were changed to always set it
according to the content of G_DEBUG in glib_init(), making the default unused.

Since nobody complained since then just remove the macro and the build option.

configure.ac

@@ -248,10 +248,6 @@ AC_ARG_ENABLE(debug,
                              [turn on debugging @<:@default=glib_debug_default@:>@]),,
               enable_debug=glib_debug_default)
 
-AC_ARG_ENABLE(gc_friendly,
-              [AS_HELP_STRING([--enable-gc-friendly],
-                              [turn on garbage collector friendliness [default=no]])],,
-              [enable_gc_friendly=no])
 AC_ARG_ENABLE(mem_pools,
               [AS_HELP_STRING([--disable-mem-pools],
 			      [disable all glib memory pools])],,
@@ -259,12 +255,6 @@ AC_ARG_ENABLE(mem_pools,
 
 GLIB_TESTS
 
-AC_MSG_CHECKING([whether to enable garbage collector friendliness])
-AS_IF([test "x$enable_gc_friendly" = "xyes"], [
-  AC_DEFINE(ENABLE_GC_FRIENDLY_DEFAULT, 1, [Whether to enable GC friendliness by default])
-  AC_MSG_RESULT([yes])
-], [ AC_MSG_RESULT([no]) ])
-
 AC_MSG_CHECKING([whether to disable memory pools])
 AS_IF([test "x$disable_mem_pools" = "xno"], [
   AC_MSG_RESULT([no])