Clean up server-kill.exp

This patch is to remove some lines which looks unnecessary.  These
lines were added when server-kill.exp was added.  In the version 1,
https://sourceware.org/ml/gdb-patches/2013-03/msg00691.html the test
calls runto_main and delete breakpoint on main,

+if ![runto_main] {
+    return -1
+}
+
+# Otherwise the breakpoint at 'main' would not cause insert
breakpoints during
+# first step.
+delete_breakpoints

However, in the version 2
https://sourceware.org/ml/gdb-patches/2013-03/msg00854.html runto_main
is removed but delete_breakpoints is still there.  AFAICS, the line of
delete_breakpoints can be removed too.

gdb/testsuite:

2014-10-11  Yao Qi  <yao@codesourcery.com>

	* gdb.server/server-kill.exp: Remove "delete_breakpoints".