support/testing: test_rust: remove kernel build

support/testing: test_rust: remove kernel build

This test already use builtin kernel provided by the testsuite infra:

    self.emulator.boot(arch="armv7",
                       kernel="builtin",
                       options=["-initrd", img])

But a second kernel is build from the its defconfig. This second kernel
is not used by the test.

The TestRust (using BR2_PACKAGE_HOST_RUST=y) is really long to build,
save some cpu time by removing the kernel build.

This unused kernel (based on 4.11.3 release) doesn't even build with
host gcc >= 10.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>