TF-M: Venv - Removes legacy support

Removes legacy support for building in default environment and switches
to only using venv, now being built directly from the TF-M dependencies.

Change-Id: I3d81cb8d85c5ec7b9314f5b14bcc7f47cefd4d6d
Signed-off-by: Matthew Dalzell <matthew.dalzell@arm.com>
2 files changed