floss: Remove rootcanal build rules

build.py contains rules for building and testing the LMP
code, that were added when it was discussed to build
the LMP for the emulator.

Now that the emulator by default builds the rust LMP
using the Cargo.toml files, these rules are no longer
necessary.

Furthermore, they pull crate dependencies from the chrome
directory, which is likely to cause issues in the future
as all other build rules for LMP use the crate versionned
from AOSP.

Test: NA
Bug: 238726856
Change-Id: I786f35939b707d5eecff4281be4fb7aee19e4e7b