remove erl_interface references

The erl_interface library has been removed in OTP 23, so these changes
remove references to this library from the project so that it can
compile.

c_src/Makefile

@@ -6,8 +6,6 @@ BASEDIR := $(abspath $(CURDIR)/..)
 PROJECT = e2qc_nif
 
 ERTS_INCLUDE_DIR ?= $(shell erl -noshell -s init stop -eval "io:format(\"~s/erts-~s/include/\", [code:root_dir(), erlang:system_info(version)]).")
-ERL_INTERFACE_INCLUDE_DIR ?= $(shell erl -noshell -s init stop -eval "io:format(\"~s\", [code:lib_dir(erl_interface, include)]).")
-ERL_INTERFACE_LIB_DIR ?= $(shell erl -noshell -s init stop -eval "io:format(\"~s\", [code:lib_dir(erl_interface, lib)]).")
 
 C_SRC_DIR = $(CURDIR)
 C_SRC_OUTPUT ?= $(CURDIR)/../priv/$(PROJECT).so
@@ -66,10 +64,10 @@ else ifeq ($(UNAME_SYS), Linux)
 	CXXFLAGS ?= -O3 -std=c++11 -finline-functions -Wall -DOS_LINUX
 endif
 
-CFLAGS += -fPIC -I $(ERTS_INCLUDE_DIR) -I $(ERL_INTERFACE_INCLUDE_DIR)
-CXXFLAGS += -fPIC -I $(ERTS_INCLUDE_DIR) -I $(ERL_INTERFACE_INCLUDE_DIR)
+CFLAGS += -fPIC -I $(ERTS_INCLUDE_DIR)
+CXXFLAGS += -fPIC -I $(ERTS_INCLUDE_DIR)
 
-LDLIBS += -L $(ERL_INTERFACE_LIB_DIR) -lerl_interface -lei
+LDLIBS += -lei
 LDFLAGS += -shared
 
 # Verbosity.