[examples] remove unneeded implementation otTaskletsSignalPending (#10849)

The implementation of `otTaskletsSignalPending` in
`examples/apps/<app>/main.c` is exactly the same as the weak
implementation in `tasklet_api.cpp`. No need to repeat it and blocking
other implementations of that function.

examples/apps/cli/main.c

@@ -60,8 +60,6 @@ OT_TOOL_WEAK void *otPlatCAlloc(size_t aNum, size_t aSize) { return calloc(aNum,
 OT_TOOL_WEAK void otPlatFree(void *aPtr) { free(aPtr); }
 #endif
 
-void otTaskletsSignalPending(otInstance *aInstance) { OT_UNUSED_VARIABLE(aInstance); }
-
 #if OPENTHREAD_POSIX && !defined(FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION)
 static otError ProcessExit(void *aContext, uint8_t aArgsLength, char *aArgs[])
 {