minor improvements of error messages of objvariants (#25040)

Because `prevFields` and `currentFields` have been already quoted by
`'`, no need to add another.

The error message was

```
The fields ''x'' and ''y'' cannot be initialized together, because they are from conflicting branches in the case object.
```

(cherry picked from commit cdb750c962)

compiler/semobjconstr.nim

@@ -236,7 +236,7 @@ proc semConstructFields(c: PContext, n: PNode, constrCtx: var ObjConstrContext,
           let prevFields = fieldsPresentInBranch(selectedBranch)
           let currentFields = fieldsPresentInBranch(i)
           localError(c.config, constrCtx.initExpr.info,
-            ("The fields '$1' and '$2' cannot be initialized together, " &
+            ("The fields $1 and $2 cannot be initialized together, " &
             "because they are from conflicting branches in the case object.") %
             [prevFields, currentFields])
           result.status = initConflict

tests/errmsgs/tconflictingfields.nim

@@ -0,0 +1,16 @@
+discard """
+  errormsg: '''
+The fields 'x' and 'y' cannot be initialized together, because they are from conflicting branches in the case object.
+'''
+"""
+
+type
+  Foo = object
+    case kind: bool
+    of true:
+      x: int
+    of false:
+      y: int
+
+
+var f = Foo(x: 1, y: 1)