libphobos: Remove undefined DRUNTIME_GC from configure scripts

libphobos/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Remove DRUNTIME_GC.

libphobos/ChangeLog

+2020-04-20  Iain Buclaw  <ibuclaw@gdcproject.org>
+
+	* configure: Regenerate.
+	* configure.ac: Remove DRUNTIME_GC.
+
 2020-04-18  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	PR d/94304

libphobos/configure

@@ -13982,7 +13982,6 @@ fi
       WERROR_FLAG="-Werror"
   fi
 
-DRUNTIME_GC
 
   druntime_target_cpu_parsed=""
   case "$target_cpu" in

libphobos/configure.ac

@@ -120,7 +120,6 @@ GCC_BASE_VER
 DRUNTIME_CONFIGURE
 DRUNTIME_MULTILIB
 DRUNTIME_WERROR
-DRUNTIME_GC
 DRUNTIME_CPU_SOURCES
 DRUNTIME_OS_SOURCES
 DRUNTIME_OS_THREAD_MODEL