examples/vdpa: remove trace of legacy linuxapp

examples/vdpa: remove trace of legacy linuxapp

This check on Linux environment has been added at a time when we already
had switched to using the boolean RTE_EXEC_ENV_LINUXAPP.
It was then missed when converting to RTE_EXEC_ENV_LINUX.

Fixes: edbed86d1cc3 ("examples/vdpa: introduce a new sample for vDPA")
Fixes: 742bde12f3bd ("build/linux: rename macro from LINUXAPP to LINUX")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Xiaolong Ye <xiaolong.ye@intel.com>