Remove unused code for old mult select design

We previously made a few CL's guarded under flags for the multi select feature. We ended up having to halt progress on this, and now that we're starting again, the design is different and we are using a new flag. Here we get rid of the unused code and flags

Bug: 277617038
Test: Verify everything still works normally, everything was guarded under flags that were off so there shouldn't be any changes
Change-Id: I2f57d1f67aa2a8cf83287f6f3df9fa6c46dbf0ab