BACKUP AND RESTORE FileGroup (PRIMARY) - WHY after restore I still see Other FileGroups?
Is there any way to restore only the [PRIMARY] filegroup?


That is what you have already done. The other filegroups exist only in metadata at this stage. Piecemeal restore allows you to restore filegroups in stages, or mark them defunct if they are no longer needed. There is no way to delete them from the system metadata though.

After the restore operation in the question, the data on the secondary filegroup is not accessible, because that filegroup has not been restored (yet). You can't see everything in the SSMS GUI only. For example:

SELECT 
    [FilegroupName] = F.name,
    F.data_space_id,
    F.is_default,
    F.is_read_only,
    DF.[file_id],
    [FileName] = DF.name,
    DF.state_desc,
    DF.is_read_only
FROM sys.filegroups AS F
JOIN sys.database_files AS DF
    ON DF.data_space_id = F.data_space_id;


The results show recovery is pending for the secondary filegroup files:




  If not, How can I get rid of the ReadOnlyFG after the restore?


You can't delete it from the metadata, but you can make it defunct by dropping it (something that is not normally possible for a filegroup that still contains files):

ALTER DATABASE PiecemealDB_NEW
REMOVE FILEGROUP ReadOnlyFG;


The query above now shows the filegroup as defunct rather than recovery pending:



Objects that were stored on the defunct filegroup cannot be accessed, modified, or dropped. The only way to get to a database state as if the defunct filegroup had never existed is to create a new database and copy the data you want to keep across to it.