Delete request if the user becomes a member by some other means

Problem/Motivation

When a user requests membership and then becomes a member by some other means, the membership request will still remain.
In this case, the membership request record will be shown in membership requests list and "Already member" is repeated!

Steps to reproduce

1. As user, request membership to a group
2. As group administrator, add previous user as member
3. See pending membership requests. The membership request done in 1 will still be shown even if the user is already a member. The operations will show twice "Already member" but no real action is possible. It is not possible to remove this record.

Proposed resolution

Remove the membership request once the user becomes a member by any means.