Remove an obsolete style block related to highlighting from `_IRCLayout.pcss` (#10451)
* Remove an obsolete style block related to highlighting from _IRCLayout.pcss
Initially it has been added with this commit 67249e1e9c17c9886812e1e2e07a71e298d2bb9e for the implementation of IRC layout, but the border declarations for highlighting were removed with this commit 3a2aa9d96cea7bedeef51f2cacf89daca3bab434 to replace the 'border-left' declaration with the 'box-shadow' declaration.
Signed-off-by: Suguru Hirahara <luixxiul@users.noreply.github.com>