Removed extension/ directory

It was not used, and also resulted in some error in compilation.
A better way to extend the code is by creating a new module directory
in src/

Makefile

 -include Makefile.conf
 
 
-SRCDIRS = src extensions test
+SRCDIRS = src test
 SUBDIRS = $(SRCDIRS) user-doc developer-doc regtest
 
 SUBDIRSCLEAN=$(addsuffix .clean,$(SUBDIRS))

extensions/DoNothing.cpp

-#include "../src/core/ActionRegister.h"
-#include "../src/core/ActionSetup.h"
-
-using namespace std;
-
-namespace PLMD{
-
-/// Action which does nothing
-///
-/// It is here to test linking of external libraries
-class DoNothing:
-  public ActionSetup
-{
-public:
-  DoNothing(const ActionOptions&ao);
-};
-
-PLUMED_REGISTER_ACTION(DoNothing,"DONOTHING")
-
-DoNothing::DoNothing(const ActionOptions&ao):
-Action(ao),
-ActionSetup(ao)
-{
-}
-
-}
-

extensions/Makefile

-
--include ../Makefile.conf
-
-SRC=$(wildcard *.cpp)
-
-OBJ=$(SRC:.cpp=.o)
-SOBJ=$(SRC:.cpp=.$(SOEXT))
-
-.PHONY: all clean
-
-ifdef GCCDEP
-all: libplumedExtensions.$(SOEXT) $(SOBJ)
-else
-all:
-	@echo No configuration available
-	@echo First run ./configure.sh in the root directory
-endif
-
-
-libplumedExtensions.$(SOEXT): $(OBJ)
-	$(LDSO) -o $@ $^
-
-clean:
-	rm -f *.o
-	rm -f *.d
-	rm -f *~
-	rm -f *.tmp
-	rm -f *.so
-	rm -f *.dylib
-
-%.$(SOEXT) : %.o
-	$(LDSO) -o $@ $^
-
-# this compiler is used for dependencies generation
-# if unset, use normal CXX
-ifndef CXXDEP
-CXXDEP=$(CXX)
-endif
-
-%.o: %.cpp
-	$(CXXDEP) -c -MM -MF$*.d $(CPPFLAGS) $(CXXFLAGS) $*.cpp
-	@cp -f $*.d $*.d.tmp
-	@sed -e 's/.*://' -e 's/\\$$//' < $*.d.tmp | fmt -1 | \
-	 sed -e 's/^ *//' -e 's/$$/:/' >> $*.d
-	@rm -f $*.d.tmp
-ifndef XLF
-	$(CXX) -c $(CPPFLAGS) $(CXXFLAGS) $*.cpp -o $*.o
-endif
-
-# Individual dependencies
-
-ifneq ($(MAKECMDGOALS),clean)
--include $(ALL_SRC_DEP)
-endif
-