Remove `rockspec:type() == "rockspec"` assertion

Nobody has claimed this yet.

I get this warning whenever I install CC:Tweaked

> Warning: The function rockspec.type() is no longer necessary and is now deprecated. Please update your plugin to remove calls to this function.

```diff
diff --git a/src/luarocks/build/lls-addon.lua b/src/luarocks/build/lls-addon.lua
index f0491dd..093c1bd 100644
--- a/src/luarocks/build/lls-addon.lua
+++ b/src/luarocks/build/lls-addon.lua
@@ -375,8 +375,6 @@ M.installAddon = installAddon
---@param noInstall boolean
---@return boolean, string?
function M.run(rockspec, noInstall)
- assert(rockspec:type() == "rockspec", "argument is not a rockspec")
-
local env = {
ABSPATH = cfg.variables["LLSADDON_ABSPATH"],
LUARCPATH = cfg.variables["LLSADDON_LUARCPATH"],
```

The only reason I wouldn't patch it now is because there are likely still users of older LuaRocks installations, which don't have this warning and might still need the assertion.

Read src/luarocks/build/lls-addon.lua, especially M.run and its rockspec argument. Check how rockspec:type() is used there and compare the expected behavior with older LuaRocks installations. Done means resolving whether the assertion can be removed without breaking supported older versions, while eliminating the deprecation warning.

Written by the indexing model from the issue text.