refactor(premeeting): remove redundant styles from prejoin.scss (#11151)
refactor(premeeting): remove redundant styles from prejoin.scss (#11151)

"prejoin-preview-dropdown-btn" and "prejoin-preview-dropdown-icon" styles are already declared using jss in DropdownButton component.
gpolitis/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet