remove testing of Tracker.jl

Author: torfjelde

src/essential/ad.jl

@@ -12,6 +12,7 @@ function _setadbackend(::Val{:forwarddiff})
     ADBACKEND[] = :forwarddiff
 end
 function _setadbackend(::Val{:tracker})
+    @warn "Usage of Tracker.jl with Turing.jl is no longer being actively tested and maintained; please use at your own risk. See Zygote.jl or ReverseDiff.jl for fully supported reverse-mode backends."
     ADBACKEND[] = :tracker
 end
 function _setadbackend(::Val{:zygote})

test/runtests.jl

@@ -65,7 +65,7 @@ macro timeit_include(path::AbstractString) :(@timeit TIMEROUTPUT $path include($
     end
 
     Turing.setrdcache(false)
-    for adbackend in (:forwarddiff, :tracker, :reversediff)
+    for adbackend in (:forwarddiff, :reversediff)
         @timeit TIMEROUTPUT "inference: $adbackend" begin
             Turing.setadbackend(adbackend)
             @info "Testing $(adbackend)"