Fix dialyze target in plugins when asdf is used

[This](ninenines/erlang.mk@e13b4c7)
`erlang.mk` commit fixed the detection of Elixir libraries for `asdf`
users, but the "old way" remained in `rabbitmq-early-plugin.mk`.
Removing the `dialyze` target env definitions fixes this issue, and, the
value of `ERL_LIBS` that is set is the same. Win-win.

Author: lukebakken

deps/rabbit_common/mk/rabbitmq-early-plugin.mk

@@ -4,9 +4,6 @@
 
 DIALYZER_OPTS ?= -Werror_handling -Wunmatched_returns -Wunknown
 
-dialyze: ELIXIR_LIBS = $(dir $(shell readlink -f `which elixir`))/../lib
-dialyze: ERL_LIBS = $(APPS_DIR):$(DEPS_DIR):$(ELIXIR_LIBS)
-
 # --------------------------------------------------------------------
 # Common Test flags.
 # --------------------------------------------------------------------