CLN: Remove int32 and float32 dtypes from IntervalTree
There isn't a practical way to actually get an `IntervalTree` with `int32`/`float32` dtypes since `IntervalIndex` is backed by two pandas indexes and we don't have a `Int32Index` or `Float32Index`. These indexes are used to create the underlying `IntervalTree` that backs an `IntervalIndex`, so we're guaranteed to have `int64`/`float64` dtype data when initializing.

The only way that comes to mind that a user could create an `IntervalTree` with `int32`/`float32` dtype would be by explicitly initializing a standalone `IntervalTree` with `int32`/`float32` arrays, which doesn't seem particularly likely.

This should also help with build times as it results in 8 less node classes being generated.

Didn't add a whatsnew note since I don't think `IntervalTree` is user facing (could be wrong?) but can add one if desired since this is technically a breaking change.
thanks @jschendel 