REORG: ebtree: clean up remains of the ebtree/ directory

The only leftovers were the unused compiler.h file and the LICENSE file
which is already mentioned in each and every ebtree file header.

A few build paths were updated in the contrib/ directory not to mention
this directory anymore, and all its occurrences were dropped from the
main makefile. From now on no other include path but include/ will be
needed anymore to build any file.
diff --git a/contrib/modsecurity/Makefile b/contrib/modsecurity/Makefile
index aa0d6e3..93119b3 100644
--- a/contrib/modsecurity/Makefile
+++ b/contrib/modsecurity/Makefile
@@ -34,7 +34,7 @@
 endif
 
 CFLAGS  += -g -Wall -pthread
-INCS += -I../../include -I../../ebtree -I$(MODSEC_INC) -I$(APACHE2_INC) -I$(APR_INC) -I$(LIBXML_INC) -I$(EVENT_INC)
+INCS += -I../../include -I$(MODSEC_INC) -I$(APACHE2_INC) -I$(APR_INC) -I$(LIBXML_INC) -I$(EVENT_INC)
 LIBS += -lpthread  $(EVENT_LIB) -levent_pthreads -lcurl -lapr-1 -laprutil-1 -lxml2 -lpcre -lyajl
 
 OBJS = spoa.o modsec_wrapper.o