[FilledInput] Simplify border overrides (#14719)
[FilledInput] Simplify border overrides (#14719)

Simple fix for #14446.  Just changed the disable css style to only specify the border bottom style, instead of the entire border property.  This will make overrides a little simpler.

Thanks!

Closes #14446.
oliviertassinari/material-ui,callemall/material-ui,kybarg/material-ui,mbrookes/material-ui,mbrookes/material-ui,allanalexandre/material-ui,mbrookes/material-ui,rscnt/material-ui,mui-org/material-ui,rscnt/material-ui,kybarg/material-ui,kybarg/material-ui,mui-org/material-ui,callemall/material-ui,kybarg/material-ui,mui-org/material-ui,oliviertassinari/material-ui,callemall/material-ui,lgollut/material-ui,rscnt/material-ui,callemall/material-ui,oliviertassinari/material-ui