Remove unnecessary ‘rustfmt’ symlink

We can instead reference the directly with a Soong reference. This is
better since we will get a hard error at build time if the reference
cannot be resolved. As an example of how this helps us, we now only
enable the host tests on the x86 architecture since we only have the
‘rustfmt’ prebuilt binary available there.

I tested this by checking that the binary is part of
‘host-unit-tests.zip’.

Test: m host-unit-tests && unzip -l out/target/product/generic_x86_64/host-unit-tests.zip | rg rustfmt
Change-Id: Icba16d7a53043483b76722b932d357084319ff34