configure: Drop --enable-githttp

Following Demi's work to use HTTPS everywhere, all users of GIT_HTTP have
been removed from the build system.  Drop the configure knob.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>

configure.ac

@@ -17,7 +17,6 @@ m4_include([m4/subsystem.m4])
 m4_include([m4/paths.m4])
 
 AX_XEN_EXPAND_CONFIG()
-AX_ARG_DEFAULT_DISABLE([githttp], [Download GIT repositories via HTTP])
 
 dnl mini-os is only ported to certain platforms
 case "$host_cpu" in