backends/xcode: simplify an obviously too-complicated function

This function has a pretty unique name, and a simple grep shows that it
is only ever called as:
```
add_comment(PbxComment('...........'))
```

It doesn't need to include logic such as handling str. Moreover it looks
like that handling was broken anyway... it handled the case where
comment is type str, by constructing a new PbxComment(str) instead of
PbxComment(comment), a condition that cannot ever be valid (and crashed
due to other assertions).

Fixes:

mesonbuild/backend/xcodebackend.py:148:42: error: Argument 1 to "PbxComment" has incompatible type "type[str]"; expected "str"  [arg-type]

mesonbuild/backend/xcodebackend.py

@@ -138,12 +138,9 @@ class PbxDict:
     def has_item(self, key):
         return key in self.keys
 
-    def add_comment(self, comment: T.Union[str, PbxComment]) -> None:
-        if isinstance(comment, str):
-            self.items.append(PbxComment(str))
-        else:
-            assert isinstance(comment, PbxComment)
-            self.items.append(comment)
+    def add_comment(self, comment: PbxComment) -> None:
+        assert isinstance(comment, PbxComment)
+        self.items.append(comment)
 
     def write(self, ofile: T.TextIO, indent_level: int) -> None:
         ofile.write('{\n')