configure: drop src/shell requirements

Problem: autotools borks because it goes looking for a Makefile for
src/shell, but support for the data-staging shell plugin was dropped
in #1102, and that directory no longer exists.

Drop this requirement from the configure script.

Author: wihobbs

configure.ac

@@ -193,7 +193,6 @@ AC_CONFIG_FILES([Makefile
   src/cmd/Makefile
   src/python/Makefile
   src/python/fluxion/Makefile
-  src/shell/Makefile
   resource/Makefile
   resource/planner/Makefile
   resource/planner/c/Makefile