selftests/exec: Remove pipe from TEST_GEN_FILES

pipe named FIFO special file is being created in execveat.c to perform
some tests. Makefile doesn't need to do anything with the pipe. When it
isn't found, Makefile generates the following build error:

make: *** No rule to make target
'../tools/testing/selftests/exec/pipe', needed by 'all'.  Stop.

pipe is created and removed during test run-time.

Amended change log to add pipe remove info:
Shuah Khan <skhan@linuxfoundation.org>

Fixes: 61016db1

 ("selftests/exec: Verify execve of non-regular files fail")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>