Revert "Set fish_clipboard_paste handler once on startup"

This reverts commit f620ddf03b.

Setting the paste handler isn't performance-sensitive.

On the other hand setting it this way makes things less transparent,
less flexible (if e.g. a paste handler is installed while the shell is running),
and causes #6286.

Fixes #6286.

[ci skip]

share/functions/fish_clipboard_paste.fish

@@ -1,24 +1,14 @@
-# Search for and set clipboard handler only once at startup
-if type -q pbpaste
-    function __fish_clipboard_paste
-        pbpaste 2>/dev/null
-    end
-else if set -q WAYLAND_DISPLAY; and type -q wl-paste
-    function __fish_clipboard_paste
-        wl-paste 2>/dev/null
-    end
-else if type -q xsel
-    function __fish_clipboard_paste
-        xsel --clipboard 2>/dev/null
-    end
-else if type -q xclip
-    function __fish_clipboard_paste
-        xclip -selection clipboard -o 2>/dev/null
-    end
-end
-
 function fish_clipboard_paste
-    set -l data (__fish_clipboard_paste)
+    set -l data
+    if type -q pbpaste
+        set data (pbpaste 2>/dev/null)
+    else if set -q WAYLAND_DISPLAY; and type -q wl-paste
+        set data (wl-paste 2>/dev/null)
+    else if type -q xsel
+        set data (xsel --clipboard 2>/dev/null)
+    else if type -q xclip
+        set data (xclip -selection clipboard -o 2>/dev/null)
+    end
 
     # Issue 6254: Handle zero-length clipboard content
     if not string match -qr . -- "$data"