[BUGFIX] Users may login with local password

LDAP users should not be allowed to authenticate using a locally-set
password. Only the LDAP password should be taken into account.

Fixes: #58933
Change-Id: Ied352c014eb1a0ed362a6ba857b527f660feec86
Reviewed-on: https://review.typo3.org/30237
Reviewed-by: Heinz Schilling
Tested-by: Heinz Schilling
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
[BUGFIX] Deleted backend users bypass check of backend.TYPO3BEUserExist

If the extension is configured to only grant access to existing backend
users, deleted backend users corresponding to LDAP users should not be
taken into account and thus not be granting access at all.

Fixes: #58932
Change-Id: I889000eb3988721acc5ff7f170de9f2e21655af6
Reviewed-on: https://review.typo3.org/30239
Reviewed-by: Heinz Schilling
Tested-by: Heinz Schilling
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers