GDScript: Remove unused switch, case and do CF keywords

They had been reserved for future implementation, but we now have the
`match` CF keyword which does the same and more.

According to @reduz `do` was even added by mistake when copying from
the shader language parser, it was never intended to add support for
`do`... `while` loops, as the syntax would be awkward in GDScript,
and the added sugar is not worth it.

Fixes #25787.

modules/gdscript/gdscript_parser.h

@@ -445,7 +445,6 @@ public:
 			CF_IF,
 			CF_FOR,
 			CF_WHILE,
-			CF_SWITCH,
 			CF_BREAK,
 			CF_CONTINUE,
 			CF_RETURN,