Remove obsolete make target

Makefile

@@ -1,6 +1,6 @@
-REBAR3 ?= rebar3
+.PHONY: all clean clean-all compile test doc dialyzer xref ci
 
-.PHONY: all clean clean_plt compile test doc dialyzer xref ci
+REBAR3 ?= rebar3
 
 all: compile xref test
 
@@ -9,11 +9,12 @@ ci: compile xref dialyzer test
 compile:
 	$(REBAR3) compile
 
-clean: clean_plt
+clean:
 	$(REBAR3) clean
 
 clean-all: clean
 	rm -rf _build
+
 test:
 	$(REBAR3) as test do eunit
 	$(REBAR3) as test do ct
@@ -24,8 +25,5 @@ xref:
 doc:
 	$(REBAR3) as docs do edoc
 
-clean_plt:
-	rm -f $(EXOMETER_PLT)
-
 dialyzer: compile
 	$(REBAR3) dialyzer