[libsigc++2] Remove the (generated) method_slot.h header.

[Murray Cumming <murrayc src gnome org>]

commit f6372ffec592f3878961a2f3c34fe284b66872e3
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Aug 12 10:03:06 2015 +0200

    Remove the (generated) method_slot.h header.
    
    Because it now contains no API.
    See https://bugzilla.gnome.org/show_bug.cgi?id=752560#c6

 sigc++/filelist.am             |    4 ++--
 sigc++/macros/method_slot.h.m4 |   23 -----------------------
 2 files changed, 2 insertions(+), 25 deletions(-)
---
diff --git a/sigc++/filelist.am b/sigc++/filelist.am
index e44fc3b..32a4f80 100644
--- a/sigc++/filelist.am
+++ b/sigc++/filelist.am
@@ -16,11 +16,11 @@
 ## along with this library.  If not, see <http://www.gnu.org/licenses/>.
 
 # Base (./)
-base_m4 = template.macros.m4 signal.h.m4 slot.h.m4 method_slot.h.m4 \
+base_m4 = template.macros.m4 signal.h.m4 slot.h.m4 \
          object_slot.h.m4 hide.h.m4 retype.h.m4 \
          limit_reference.h.m4
 base_built_cc =
-base_built_h = signal.h slot.h method_slot.h \
+base_built_h = signal.h slot.h \
               object_slot.h hide.h retype.h limit_reference.h
 
 # Functors (functors/)