Fix RichTextLabel bug which clears its content when theme changed

[Chaosus]

I found this bug when working with RichTextLabel for chat in my game, it clears its content every time theme or style changed, I dont wanna touch the work logic of parse_bbcode so instead removing clear from it, I just remove this unnecessary notification, which called it and breaks a chat logic

[akien-mga]

This code was added by @reduz in 93ce7d9, so it would be good to know why it was needed back then before removing it.

[Chaosus]

Its 3 years old code, and he can did some mistakes, the theme notification here is invalid by default, cuz it calls parse_bbcode which calls clear every time the theme or style changed(if bbcode enabled). I removed whole notification cuz its old and invalid and better be rewrited from scratch..

PS Ok, lets wait for @reduz review

[akien-mga]

As discussed on IRC, this was due to a misuse of set_use_bbcode due to a confusing API (to be changed as part of #16863). For the time being it will be better documented in godotengine/godot-docs#1466.