Remove redundant badges from readme
Remove redundant badges from readme

* Codecov is only tracking JS coverage, so displaying a coverage figure for the whole project is misleading
* wagtail-slack.now.sh appears to be dead

zerolab/wagtail,zerolab/wagtail,mixxorz/wagtail,thenewguy/wagtail,rsalmaso/wagtail,zerolab/wagtail,thenewguy/wagtail,mixxorz/wagtail,wagtail/wagtail,rsalmaso/wagtail,zerolab/wagtail,mixxorz/wagtail,gasman/wagtail,zerolab/wagtail,mixxorz/wagtail,jnns/wagtail,gasman/wagtail,wagtail/wagtail,torchbox/wagtail,thenewguy/wagtail,thenewguy/wagtail,torchbox/wagtail,gasman/wagtail,jnns/wagtail,rsalmaso/wagtail,torchbox/wagtail,jnns/wagtail,thenewguy/wagtail,jnns/wagtail,rsalmaso/wagtail,torchbox/wagtail,mixxorz/wagtail,wagtail/wagtail,gasman/wagtail,rsalmaso/wagtail,gasman/wagtail,wagtail/wagtail,wagtail/wagtail