Clean up gdb.gdb/selftest.exp

Clean up gdb.gdb/selftest.exp

I recently see the test fails like this,

(gdb) PASS: gdb.gdb/selftest.exp: step over argv initialization
list^M
487       std::vector<struct cmdarg> cmdarg_vec;^M
(gdb) FAIL: gdb.gdb/selftest.exp: unknown source line (after step over argv initialization)

step^M
std::vector<cmdarg, std::allocator<cmdarg> >::vector (this=0x7fffffffdc10) at ../../binutils-gdb/gdb/main.c:487^M
487       std::vector<struct cmdarg> cmdarg_vec;^M
(gdb) FAIL: gdb.gdb/selftest.exp: step into xmalloc call

These fails are caused by using std::vector in commit
f60ee22ea1d62f7004511ec65a3ad76890032d88.  selttest.exp should match
the source code of GDB.  It is a maintenance pain, so this patch
removes do_steps_and_nexts.

gdb/testsuite:

2016-12-19  Yao Qi  <yao.qi@linaro.org>

	* gdb.gdb/selftest.exp (do_steps_and_nexts): Remove.
	(test_with_self): Don't call do_steps_and_nexts, and remove
	code about stepping into xmalloc.