FINERACT-2722: Remove orphaned CashierWritePlatformService and ModifyCashierCommandHandler

[AshharAhmadKhan]

Description

Removes two dead classes from the teller/cashier module and one stale
javadoc reference.

CashierWritePlatformService
(fineract-branch/src/main/java/org/apache/fineract/organisation/teller/service/)
is a plain concrete class with no @Service or @Component
annotation. All three of its methods (allocateCashierToTeller,
deleteCashier, modifyCashier) are null stubs with no
implementation.

ModifyCashierCommandHandler
(fineract-branch/src/main/java/org/apache/fineract/organisation/teller/handler/)
injects CashierWritePlatformService through a constructor field and
calls modifyCashier on it. This handler has no @Service and no
@CommandType annotation, so Fineract's command dispatcher never
routes anything to it. Nothing else in the codebase constructs or
injects this handler.

Also removed a stale @see javadoc line in
DeleteCashierAllocationCommandHandler.java, which is live and
correctly wired to TellerWritePlatformService. The line pointed at
CashierWritePlatformService, which it never actually depended on.

Why this is safe

The live cashier-teller allocation path is
AllocateCashierToTellerCommandHandler, which calls
TellerWritePlatformService.allocateCashierToTeller(Long entityId, JsonCommand command),
implemented in TellerWritePlatformServiceJpaImpl.java line 197.
This is a separate class from CashierWritePlatformService that
happens to share method names.

Verification

• Full-repo grep across .java, .xml, .yml, .yaml, .properties
  for both class names. Only the two deleted files and the one stale
  doc comment matched.
• Permission table grep for MODIFYCASHIER / MODIFY_CASHIER. No
  results, no dangling m_permission row.
• Reflection-based lookup grep (Class.forName, getBean). No
  results.
• Test file grep across the repo (--include="*Test.java"). No
  results, no test references either class.

[AshharAhmadKhan]

hey @adamsaghy , failure look flaky, can you please retrigger those checks? thanks a lot!