Remove github issues badge.
Remove github issues badge.

It frequently fails to appear and I don't think it actually gains us
anything.

ahill818/MetPy,dopplershift/MetPy,ahaberlie/MetPy,jrleeman/MetPy,dopplershift/MetPy,jrleeman/MetPy,ahaberlie/MetPy,Unidata/MetPy,Unidata/MetPy,deeplycloudy/MetPy,ShawnMurd/MetPy