uEnv.txt was needed due to a bug in the default U-Boot environment,
this has been fixed upstream and the TI repo used in this build.
Remove this file.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>