test: enable JIT for Lua Fun chain iterator back

This patch reverts the temporary fix introduced in commit
5fa7ded ("test: disable JIT for Lua Fun
chain iterator") since the issues with invalid traces generation for
<fun.chain> iterator are resolved and JIT can be enabled back then.

Follows up #4252
Follows up #5118

Signed-off-by: Igor Munkin <imun@tarantool.org>

test/box-tap/key_def.test.lua

@@ -5,13 +5,6 @@ local ffi = require('ffi')
 local json = require('json')
 local fun = require('fun')
 
--- XXX fix for gh-4252: to prevent invalid trace assembling (see
--- https://github.com/LuaJIT/LuaJIT/issues/584) disable JIT for
--- <fun.chain> iterator (i.e. <chain_gen_r1>). Since the function
--- is local, the dummy chain generator is created to obtain the
--- function GC object.
-jit.off(fun.chain({}).gen)
-
 local key_def_lib = require('key_def')
 
 local usage_error = 'Bad params, use: key_def.new({' ..