Remove redundant text in @deprecated messages (#6876)

ExDoc renders "This function is deprecated. {...}.", so messages should
not repeat "deprecated" nor end in punctuation.

Using MFA instead of function call syntax to refer to an alternative
function, e.g. `get_flash/2`, is idiomatic and consistent.

Do not touch messages in `@moduledoc false` modules or `@doc false`
functions, as they are not visible in HexDocs anyway, thus avoiding
unnecessary churn.

Author: rhcarvalho

lib/phoenix/controller.ex

@@ -1714,7 +1714,7 @@ defmodule Phoenix.Controller do
       %{"info" => "Welcome Back!"}
 
   """
-  @deprecated "get_flash/1 is deprecated. Use the @flash assign provided by the :fetch_flash plug"
+  @deprecated "Use the @flash assign provided by the :fetch_flash plug"
   def get_flash(conn) do
     Map.get(conn.assigns, :flash) ||
       raise ArgumentError, message: "flash not fetched, call fetch_flash/2"
@@ -1730,7 +1730,7 @@ defmodule Phoenix.Controller do
       "Welcome Back!"
 
   """
-  @deprecated "get_flash/2 is deprecated. Use Phoenix.Flash.get(@flash, key) instead"
+  @deprecated "Use Phoenix.Flash.get/2 instead"
   def get_flash(conn, key) do
     get_flash(conn)[flash_key(key)]
   end

lib/phoenix/test/conn_test.ex

@@ -274,14 +274,14 @@ defmodule Phoenix.ConnTest do
   Gets the whole flash storage.
   """
   @spec get_flash(Conn.t) :: map
-  @deprecated "get_flash/1 is deprecated. Use conn.assigns.flash instead"
+  @deprecated "Use conn.assigns.flash instead"
   def get_flash(conn), do: conn.assigns.flash
 
   @doc """
   Gets the given key from the flash storage.
   """
   @spec get_flash(Conn.t, term) :: term
-  @deprecated "get_flash/2 is deprecated. Use Phoenix.Flash.get/2 instead"
+  @deprecated "Use Phoenix.Flash.get/2 instead"
   def get_flash(conn, key) do
     Phoenix.Flash.get(conn.assigns.flash, key)
   end