clean linker script

riscv-rt/link.x.in

@@ -43,13 +43,10 @@ PROVIDE(InstructionPageFault = ExceptionHandler);
 PROVIDE(LoadPageFault = ExceptionHandler);
 PROVIDE(StorePageFault = ExceptionHandler);
 
-PROVIDE(UserSoft = DefaultHandler);
 PROVIDE(SupervisorSoft = DefaultHandler);
 PROVIDE(MachineSoft = DefaultHandler);
-PROVIDE(UserTimer = DefaultHandler);
 PROVIDE(SupervisorTimer = DefaultHandler);
 PROVIDE(MachineTimer = DefaultHandler);
-PROVIDE(UserExternal = DefaultHandler);
 PROVIDE(SupervisorExternal = DefaultHandler);
 PROVIDE(MachineExternal = DefaultHandler);