samples: tfm_secure_partition: Remove conditional from partition

Remove the conditional attribute from the sample partition manifest.
The conditional behavior will change with TF-M 1.5 to only accept
cmake bool values on/off enabled/disabled true/false and is intended
to be generated by the build system.
Since the partition is supposed to always be enabled in the sample
there is no need to have a conditional for it.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>