Fix read after xfree in linux-nat.c:linux_nat_detach

Fix read after xfree in linux-nat.c:linux_nat_detach

At the end of linux_nat_detach the main_lwp is deleted (delete_lwp).
This is problematic as during detach (detach_one_lwp and
linux_fork_detach) main_lwp already gets freed.  Thus calling
delete_lwp causes a read after free.  Fix it by removing the
unnecessary delete_lwp.

gdb/ChangeLog:
2017-04-11  Philipp Rudo  <prudo@linux.vnet.ibm.com>

	* linux-nat.c (linux_nat_detach): Remove delete_lwp call.