selftests/coredump: Remove the read() that fails the test

selftests/coredump: Remove the read() that fails the test

Resolve a conflict between
  commit 6a68d28066b6 ("selftests/coredump: Fix "socket_detect_userspace_client" test failure")
and
  commit 994dc26302ed ("selftests/coredump: fix build")

The first commit adds a read() to wait for write() from another thread to
finish. But the second commit removes the write().

Now that the two commits are in the same tree, the read() now gets EOF and
the test fails.

Remove this read() so that the test passes.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/20250811074957.4079616-1-namcao@linutronix.de
Signed-off-by: Christian Brauner <brauner@kernel.org>