No need to forward-declare tls_atom_version() type

It was needed to fix the build with Erlang R16B03. Now that the minimum
version is Erlang 18.3 in master, we can revert what was done in
c9a9a9b.

[#118562897]

Makefile

@@ -24,16 +24,6 @@ include erlang.mk
 include mk/rabbitmq-build.mk
 include mk/rabbitmq-dist.mk
 
-# --------------------------------------------------------------------
-# Compilation.
-# --------------------------------------------------------------------
-
-# $(ERTS_VER) is set in `rabbitmq-build.mk` above.
-tls_atom_version_MAX_ERTS_VER = 6.0
-ifeq ($(call compare_version,$(ERTS_VER),$(tls_atom_version_MAX_ERTS_VER),<),true)
-RMQ_ERLC_OPTS += -Ddefine_tls_atom_version
-endif
-
 # --------------------------------------------------------------------
 # Framing sources generation.
 # --------------------------------------------------------------------

mk/rabbitmq-build.mk

@@ -41,10 +41,6 @@ lager_extra_sinks = $(subst $(space),$(comma),$(LAGER_EXTRA_SINKS))
 RMQ_ERLC_OPTS += +'{parse_transform,lager_transform}' \
 		 +'{lager_extra_sinks,[$(lager_extra_sinks)]}'
 
-# Erlang R16B03 has no support for new types in Erlang 17.0, leading to
-# a build-time error.
-ERTS_VER := $(shell erl -version 2>&1 | sed -E 's/.* version //')
-
 # Push our compilation options to both the normal and test ERLC_OPTS.
 ERLC_OPTS += $(RMQ_ERLC_OPTS)
 TEST_ERLC_OPTS += $(RMQ_ERLC_OPTS)

src/rabbit_net.erl

@@ -17,13 +17,6 @@
 -module(rabbit_net).
 -include("rabbit.hrl").
 
--ifdef(define_tls_atom_version).
-%% In Erlang R16B03, tls_atom_version() is defined in ssl_internal.hrl,
-%% which is not included by ssl_api.hrl. Instead of including it here,
-%% we redefine it to avoid too much pollution.
--type tls_atom_version() :: sslv3 | tlsv1 | 'tlsv1.1' | 'tlsv1.2'.
--endif.
-
 -include_lib("ssl/src/ssl_api.hrl").
 
 -export([is_ssl/1, ssl_info/1, controlling_process/2, getstat/2,