kconfig: drop T_WORD from the RHS of 'prompt' symbol

Commit 8636a1f9 ("treewide: surround Kconfig file paths with double
quotes") killed use-cases to reduce an unquoted string into the 'prompt'
symbol.

Kconfig still allows to use an unquoted string in the context of menu,
source, or prompt.

So, you can omit quoting if the prompt is a single word:

    bool foo

..., but I do not think this is so useful.

Let's require quoting:

    bool "foo"

All the Kconfig files in the kernel are written in this way.

Remove the T_WORD from the right-hand side of the symbol 'prompt'.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/parser.y

@@ -434,9 +434,7 @@ prompt_stmt_opt:
 	menu_add_prompt(P_PROMPT, $1, $2);
 };
 
-prompt:	  T_WORD
-	| T_WORD_QUOTE
-;
+prompt:	  T_WORD_QUOTE
 
 end:	  T_ENDMENU T_EOL	{ $$ = "menu"; }
 	| T_ENDCHOICE T_EOL	{ $$ = "choice"; }