fish integration: drop redundant OSC 133 markers in upcoming fish 3.8

The upcoming fish 3.8 release will output OSC 133 sequences
unconditionally [1].

I tested ctrl-shift-{g,x,z} bindings both without and with kitty's
shell integration on top; everything seems to work.

Let's simplify kitty integration by removing the markers for the
upcoming fish >= 3.8.

I have hopes that the native OSC 133 implementation address #7200
though I'm not sure if I could reproduce this bug (I only saw a
similar bug when `fish_handle_reflow` was not enabled, which fish
also does now (same commit)).
cc @iacore let me know if you can reproduce #7200 with latest fish master.

[1]: 3b9e3e251b

docs/shell-integration.rst

@@ -85,6 +85,8 @@ no-cwd
 no-prompt-mark
     Turn off marking of prompts. This disables jumping to prompt, browsing
     output of last command and click to move cursor functionality.
+    Note that for the fish shell this does not take effect, since fish always
+    marks prompts.
 
 no-complete
     Turn off completion for the kitty command.