Change bp2build to reflect arm_isa refactor

We now rely on a `with_features` clause to effectively disable
thumb when `arm_isa_arm` is enabled. Thus, we no longer need to
add `-arm_isa_thumb` in bp2build.

Bug: 261733820
Test: Unit tests
Change-Id: I9da85834855b297a75097ce3a11e101f55bd53f4