refactor(options): always allocate option values (#30917)

Instead of keeping `P_ALLOCED` and `P_DEF_ALLOCED` flags to check if an
option value is allocated, always allocate option values to simplify the
logic.

Ref: #25672

src/nvim/generators/gen_options.lua

@@ -67,7 +67,6 @@ local function get_flags(o)
     end
   end
   for _, flag_desc in ipairs({
-    { 'alloced' },
     { 'nodefault' },
     { 'no_mkrc' },
     { 'secure' },