Remove unused top level files

I don't think anyone really uses the tools that uses these files these
days, and they are another thing that needs to be updated every so
often.

Procfile

@@ -1,2 +0,0 @@
-web: mix phx.server
-release: mix ecto.migrate

coveralls.json

@@ -1,7 +0,0 @@
-{
-  "skip_files": [
-    "test/support",
-    "lib/mix/tasks/pleroma/benchmark.ex",
-    "lib/credo/check/consistency/file_location.ex"
-  ]
-}

elixir_buildpack.config

@@ -1,2 +0,0 @@
-elixir_version=1.14.3
-erlang_version=25.3