utils.bbclass: remove the unused oe_system

utils.bbclass: remove the unused oe_system

The following functions in meta/classes/utils.bbclass were not
used by anyone, and they didn't work when I tried to use them:

def oe_popen_env(d):
def oe_run(d, cmd, **kwargs):
def oe_popen(d, cmd, **kwargs):
def oe_system(d, cmd, **kwargs):

There error was:
AttributeError: type object 'str' has no attribute 'getVar'

We have bitbake/lib/bb/process.py to instead, so remove them.

[YOCTO #2489]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>