Remove stale path validation logic

We used to warn about PATH and CDPATH that are not valid directories,
but only if they contain colons.
However, the warning was a false positive because we would split
those values by colons anyway. So there is nothing left we want to
warn about.

Fixes #8095

tests/checks/set.fish

@@ -715,3 +715,13 @@ set --show ""
 #CHECKERR: set --show ""
 #CHECKERR: ^
 #CHECKERR: (Type 'help set' for related documentation)
+
+# Test path splitting
+begin
+    set -l PATH /usr/local/bin:/usr/bin
+    echo $PATH
+    # CHECK: /usr/local/bin /usr/bin
+    set -l CDPATH .:/usr
+    echo $CDPATH
+    # CHECK: . /usr
+end