os: use unlink() to remove file (#9220)

removeFile() behavior should now be consistant between Windows and POSIX

Fixes #9200

tests/stdlib/tos.nim

@@ -39,6 +39,7 @@ false
 true
 true
 Raises
+Raises
 true
 true
 true
@@ -119,6 +120,14 @@ except IOError:
   echo "Raises"
 removeFile(dname)
 
+# removeFile should not remove directory
+createDir(dname)
+try:
+  removeFile(dname)
+except OSError:
+  echo "Raises"
+removeDir(dname)
+
 # test copyDir:
 createDir("a/b")
 open("a/b/file.txt", fmWrite).close