Under Tasks>Age Group Links the target groups for Gentlemen Master and Ladies Master are given as Men and Women respectively, not 50+. These seem not to be correctable by the user.
Check the lookups in the Age Groups table (Manage > Age Groups). It is probable that these age groups have the lookup set to Men/Women. This will be why they are not editable. If you change the lookup value to Men 50+/Women 50+ you should be able to delete the link from the links table.
Note the function of the age group links is to alias one age group to another for the purpose of Club Records. Linking Gentlemen Master to Men means that that for club record purposes Gentlemen Master is considered to be the same as Men.
The lookup value is used to retrieve handicaps and classifications from the tables.