testsuite, gdb.btrace: remove assembly-check in delta.exp

testsuite, gdb.btrace: remove assembly-check in delta.exp

In gdb.btrace/delta.exp, we test that we do not extend the trace
unintentionally.  This can be tested by checking the number of
instructions.

If we wanted to check the instruction history, as well, we'd need to work
on an assembly file to have deterministic behaviour.  This isn't really
necessary for this test, however, and covered elsewhere.  Also remove the
function call history check for the same reason.