Revert "mptcp: add data lock for sk timers"

This reverts commit 4293248c.

Additional locks are not needed, all the touched sections
are already under mptcp socket lock protection.

Fixes: 4293248c ("mptcp: add data lock for sk timers")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>