Remove 'Switch to plain text editor' in CKEdtior
You do not need to use CSS. There is an option within the CKEditor profile. 

Go to Configuration > Content Authoring > CKEditor then select edit next to the profile you want to effect. This way you can also limit this too certain users. 

Then select editor appearance and scroll down to "Show the disable/enable rich text editor toggle" then select hide and save the profile.