Remove syntax checking feature preview from README

It's not available due to changes in flycheck code, which is the actual
component handling the visual markerks and automation. I figured it's
not fair to advertise a feature this plugin doesn't even have.

See this ticket for investigation:

Latest version of Flycheck incompatible with Omnisharp checker · Issue
https://github.com/sp3ctum/omnisharp-emacs/issues/66
Bump version 1.7 -> 1.8