MLECO-4246: Bug fix MLEK hardfault when SRAM exceeded

GCC builds of inference runner hardfault when tensor allocation
fails.

Traced to duplicated delete calls on interpreter member variable in app
code and ~Model() which has undefined behaviour

Signed-off-by: Liam Barry <liam.barry@arm.com>
Change-Id: I3872338b4fdc3c25aa26a5cd55991cd575da5a3c