test: app/socket flaky fails at 1118 line

Found that on high loaded hosts the test flaky fails at:

[004] --- app/socket.result	Mon Jul 15 07:18:57 2019
[004] +++ app/socket.reject	Tue Jul 16 16:37:35 2019
[004] @@ -1118,7 +1118,7 @@
[004]  ...
[004]  ch:get(1)
[004]  ---
[004] -- true
[004] +- null
[004]  ...
[004]  s:error()
[004]  ---

Found that the test in previous was used for testing the
the channel get() function timeout and the error occurred
on it, but later the checking error changed to:
"builtin/socket.lua: attempt to use closed socket" and the
test became not correct. Because for now it passes when the
socket read function runs before the socket closing, but in
this way read call doesn't wait. In the other way on high
loaded hosts the close call may occure before read call and
in this way read call halts and socket get call returns
'null'. As seen both ways are not correct to check the error.
Decided to remove this subtest.

Check commit ba7a4fee ("Add tests for socket:close closes #360")

Fixes #4354