weechat: do not manually set TERM in test

No longer necessary since https://github.com/Homebrew/brew/pull/982.

Closes #4941.

Signed-off-by: Zhiming Wang <zmwangx@gmail.com>

Formula/weechat.rb

@@ -59,7 +59,6 @@ class Weechat < Formula
   end
 
   test do
-    ENV["TERM"] = "xterm"
     system "weechat", "-r", "/quit"
   end
 end